ManilaShares.create_and_delete_share failes with existing_users and manila_share_networks context

deployment:
-----------
{
    "type": "ExistingCloud",
    "auth_url": "http://127.0.0.1:5000/v2.0",
    "region_name": "RegionOne",
    "endpoint_type": "public",
    "admin": {
        "username": "admin",
        "password": "password",
        "tenant_name": "demo"
    },
    "users": [
        {
            "username": "demo",
            "password": "password",
            "tenant_name": "demo"
        }
    ]
}

task:
-------

{
    "ManilaShares.create_and_delete_share": [
        {
            "args": {
                "share_proto": "nfs",
                "size": 1,
                "min_sleep": 0,
                "max_sleep": 0,
            },
            "runner": {
                "type": "constant",
                "times": 1,
                "concurrency": 1
            },
            "context": {
                "quotas": {
                    "manila": {
                        "shares": -1,
                        "gigabytes": -1
                    }
                },
                "manila_share_networks": {
                    "use_share_networks": True,
                    "share_networks": {
                        "demo": ["themnet"]
                    }
                }
            }
        }
    ]
}

result:
-------

[...]
2016-05-30 21:33:23.445 47995 INFO rally.plugins.openstack.context.keystone.existing_users [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Starting: Enter context: `existing_users`
2016-05-30 21:33:23.575 47995 INFO rally.plugins.openstack.context.keystone.existing_users [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Completed: Enter context: `existing_users`
2016-05-30 21:33:23.575 47995 INFO rally.plugins.openstack.context.quotas.quotas [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Starting: Enter context: `quotas`
2016-05-30 21:33:24.169 47995 INFO rally.plugins.openstack.context.quotas.quotas [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Completed: Enter context: `quotas`
2016-05-30 21:33:24.170 47995 INFO rally.plugins.openstack.context.manila.manila_share_networks [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Starting: Enter context: `manila_share_networks`
2016-05-30 21:33:24.563 47995 INFO rally.plugins.openstack.context.manila.manila_share_networks [-] share network found: 'themnet', 'None'
2016-05-30 21:33:24.564 47995 INFO rally.plugins.openstack.context.manila.manila_share_networks [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Completed: Enter context: `manila_share_networks`
Process Process-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/opt/stack/rally/rally/plugins/common/runners/constant.py", line 74, in _worker_process
    scenario_context = runner._get_scenario_context(iteration, context)
  File "/opt/stack/rally/rally/task/runner.py", line 48, in _get_scenario_context
    context_obj = copy.deepcopy(context_obj)
  File "/usr/lib/python2.7/copy.py", line 163, in deepcopy
    y = copier(x, memo)
  File "/usr/lib/python2.7/copy.py", line 257, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/lib/python2.7/copy.py", line 163, in deepcopy
    y = copier(x, memo)
  File "/usr/lib/python2.7/copy.py", line 257, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/lib/python2.7/copy.py", line 163, in deepcopy
    y = copier(x, memo)
  File "/usr/lib/python2.7/copy.py", line 257, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/lib/python2.7/copy.py", line 163, in deepcopy
    y = copier(x, memo)
  File "/usr/lib/python2.7/copy.py", line 257, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/lib/python2.7/copy.py", line 190, in deepcopy
    y = _reconstruct(x, rv, 1, memo)
  File "/usr/lib/python2.7/copy.py", line 334, in _reconstruct
    state = deepcopy(state, memo)
  File "/usr/lib/python2.7/copy.py", line 163, in deepcopy
    y = copier(x, memo)
  File "/usr/lib/python2.7/copy.py", line 257, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/lib/python2.7/copy.py", line 182, in deepcopy
    rv = reductor(2)
  File "/usr/lib/python2.7/multiprocessing/sharedctypes.py", line 213, in __reduce__
    assert_spawning(self)
  File "/usr/lib/python2.7/multiprocessing/forking.py", line 52, in assert_spawning
    ' through inheritance' % type(self).__name__
RuntimeError: Synchronized objects should only be shared between processes through inheritance
2016-05-30 21:33:24.586 47995 INFO rally.plugins.openstack.context.cleanup.user [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Starting: user resources cleanup
2016-05-30 21:33:26.578 47995 INFO rally.plugins.openstack.context.cleanup.user [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Completed: user resources cleanup
2016-05-30 21:33:26.579 47995 INFO rally.plugins.openstack.context.manila.manila_share_networks [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Starting: Exit context: `manila_share_networks`
2016-05-30 21:33:26.579 47995 INFO rally.plugins.openstack.context.manila.manila_share_networks [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Completed: Exit context: `manila_share_networks`
2016-05-30 21:33:26.580 47995 INFO rally.plugins.openstack.context.quotas.quotas [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Starting: Exit context: `quotas`
2016-05-30 21:33:26.696 47995 INFO rally.plugins.openstack.context.quotas.quotas [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Completed: Exit context: `quotas`
2016-05-30 21:33:26.697 47995 INFO rally.plugins.openstack.context.keystone.existing_users [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Starting: Exit context: `existing_users`
2016-05-30 21:33:26.697 47995 INFO rally.plugins.openstack.context.keystone.existing_users [-] Task 864e0db9-c600-4b5c-9be5-943cccdb292c | Completed: Exit context: `existing_users`
[...]