[SRU] glance do not require hypervisor_mapping

[Impact]
currently the glance mirror requires hypervisor_mapping config in the api.
Better to not required that as a library consumer would not necessarily provide it.

[Test Case]

* deploy a openstack environment with keystone v2 enabled
  - get a copy of the bundle available at http://paste.ubuntu.com/p/qxwSDtDZ52/ , this bundle deploys a minimal version of xenial-mitaka.

Expected Result:
- "glance image-list" lists trusty and xenial images
- the file glance-simplestreams-sync/0:/var/log/glance-simplestreams-sync.log contains details of the images pulled from cloud-images.u.c (example: https://pastebin.ubuntu.com/p/RWG8QrkVDz/ )

Actual result:

- "glance image-list" is empty
- glance-simplestreams-sync/0 is in blocked state and message "Image sync failed, retrying soon."

In /var/log/glance-simplestreams-sync.log:
ERROR * 02-14 15:46:07 [PID:1898] * root * Exception during syncing:
Traceback (most recent call last):
  File "/usr/share/glance-simplestreams-sync/glance-simplestreams-sync.py", line 462, in main
    do_sync(charm_conf, status_exchange)
  File "/usr/share/glance-simplestreams-sync/glance-simplestreams-sync.py", line 273, in do_sync
    tmirror.sync(smirror, path=initial_path)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 91, in sync
    return self.sync_index(reader, path, data, content)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 254, in sync_index
    self.sync(reader, path=epath)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 89, in sync
    return self.sync_products(reader, path, data, content)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 341, in sync_products
    self.insert_item(item, src, target, pgree, ipath_cs)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/glance.py", line 242, in insert_item
    if self.config['hypervisor_mapping'] and 'ftype' in flat:
KeyError: 'hypervisor_mapping'

[Regression Potential]

* This patch makes an argument optional only, there is no expected regressions in users of this library.

[Other Info]

The bundle used in the test case uses a modified version of the glance-simplestreams-sync charm that removes the hypervisor_mapping parameter when using simplestreams library. https://pastebin.ubuntu.com/p/Ny7jFnGfnY/