typo in IVS related privsep method, VM creation failure

Description
===========
As part of the change here [1], a few calls to edit files as root were moved to a privsep class that encompasses all those edits.
A typo creeped in, causing a failure when creating a VM with VIF type IVS.

Method in question:
@nova.privsep.dacnet_admin_pctxt.entrypoint
def disable_ipv6(interface):
    """Disable ipv6 for a bridge."""
    with open('/proc/sys/net/ipv6/conf/%s/disable_ipv' % interface, 'w') as f:
        f.write('1')

The name of the file should be 'disable_ipv6' instead of 'disable_ipv'.
Missing '6' at the end.

Logs & Configs
==============
log excerpt for failure:
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [req-e2560c77-8950-4dc6-8f4f-44254b184bbb 17bbd148d3f34a42bede6bdb8fcc84aa 62daca4c6dff406685f3fcf754cb82c4 - default default] [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] Instance failed to spawn: IOError: [Errno 2] No such file or directory
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] Traceback (most recent call last):
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 2236, in _build_resources
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] yield resources
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 2016, in _build_and_run_instance
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] block_device_info=block_device_info)
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 3100, in spawn
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] destroy_disks_on_failure=True)
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 5590, in _create_domain_and_network
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] destroy_disks_on_failure)
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] self.force_reraise()
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] six.reraise(self.type_, self.value, self.tb)
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 5549, in _create_domain_and_network
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] self.plug_vifs(instance, network_info)
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 829, in plug_vifs
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] self.vif_driver.plug(instance, vif)
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/vif.py", line 896, in plug
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] func(instance, vif)
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/vif.py", line 722, in plug_ivs
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] self.plug_ivs_hybrid(instance, vif)
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/vif.py", line 703, in plug_ivs_hybrid
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] nova.privsep.libvirt.disable_ipv6(br_name)
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] File "/usr/lib/python2.7/site-packages/oslo_privsep/priv_context.py", line 207, in _wrap
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] return self.channel.remote_call(name, args, kwargs)
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] File "/usr/lib/python2.7/site-packages/oslo_privsep/daemon.py", line 202, in remote_call
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] raise exc_type(*result[2])
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d] IOError: [Errno 2] No such file or directory
2018-09-24 14:21:47.167 1 ERROR nova.compute.manager [instance: a631eff3-9d68-4e06-a7ce-4338e29e8e2d]

I'll open a fix PR as well. And add a check that file exists before editing - apparently that's not true for this particular case and wasn't carried over from existing code.

Note: only affects stable/queens branch. Later releases has proper calling to os-vif-bigswitch and this code has been cleaned up from Nova.

[1] https://review.openstack.org/#/c/489438/