lxd charm fails when trying to write to RO filesystem

[Adam Stokes]

Reproducer:

conjure-up -d openstack-novalxd localhost

Problem:

all services are already in containers so trying to write to /sys fails since it's RO.

Error:

The /var/log/juju/unit-lxd-0.log on the unit reports:
2016-10-21 01:09:33 INFO config-changed Traceback (most recent call last):
2016-10-21 01:09:33 INFO config-changed File "/var/lib/juju/agents/unit-lxd-0/charm/hooks/config-changed", line 140, in <module>
2016-10-21 01:09:33 INFO config-changed main()
2016-10-21 01:09:33 INFO config-changed File "/var/lib/juju/agents/unit-lxd-0/charm/hooks/config-changed", line 134, in main
2016-10-21 01:09:33 INFO config-changed hooks.execute(sys.argv)
2016-10-21 01:09:33 INFO config-changed File "/var/lib/juju/agents/unit-lxd-0/charm/hooks/charmhelpers/core/hookenv.py", line 715, in execute
2016-10-21 01:09:33 INFO config-changed self._hooks[hook_name]()
2016-10-21 01:09:33 INFO config-changed File "/var/lib/juju/agents/unit-lxd-0/charm/hooks/config-changed", line 78, in config_changed
2016-10-21 01:09:33 INFO config-changed configure_lxd_host()
2016-10-21 01:09:33 INFO config-changed File "/var/lib/juju/agents/unit-lxd-0/charm/hooks/charmhelpers/core/decorators.py", line 40, in _retry_on_exception_inner_2
2016-10-21 01:09:33 INFO config-changed return f(*args, **kwargs)
2016-10-21 01:09:33 INFO config-changed File "/var/lib/juju/agents/unit-lxd-0/charm/hooks/lxd_utils.py", line 429, in configure_lxd_host
2016-10-21 01:09:33 INFO config-changed with open(EXT4_USERNS_MOUNTS, 'w') as userns_mounts:
2016-10-21 01:09:33 INFO config-changed IOError: [Errno 30] Read-only file system: '/sys/module/ext4/parameters/userns_mounts'
2016-10-21 01:09:33 ERROR juju.worker.uniter.operation runhook.go:107 hook "config-changed" failed: exit status 1

proposed fix:

https://review.openstack.org/#/c/389740/

This currently breaks _all_ OpenStack deployments on a single system.