unit tests don't pass due to c-h sync bringing in 'apt' compatibility

[Alex Kavanagh]

Essentially, the types have changed a bit, and so the compatibility isn't exact. Ought to fix this in c-h, but will patch in ceph-proxy:

unit_tests.test_ceph_hooks.TestHooks.test_client_relation_joined
----------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    b'Traceback (most recent call last):'
    b' File "/home/alex/Projects/Canonical/git/opendev.org/openstack/charm-ceph-proxy/hooks/ceph.py", line 89, in get_version'
    b' pkg = cache[package]'
    b' File "/home/alex/Projects/Canonical/git/opendev.org/openstack/charm-ceph-proxy/charmhelpers/fetch/ubuntu_apt_pkg.py", line 71, in __getitem__'
    b' apt_result = self._apt_cache_show([package])[package]'
    b' File "/home/alex/Projects/Canonical/git/opendev.org/openstack/charm-ceph-proxy/charmhelpers/fetch/ubuntu_apt_pkg.py", line 162, in _apt_cache_show'
    b" if line.startswith(' '):"
    b'TypeError: startswith first arg must be bytes or a tuple of bytes, not str'
    b''
    b'During handling of the above exception, another exception occurred:'
    b''
    b'Traceback (most recent call last):'
    b' File "/home/alex/Projects/Canonical/git/opendev.org/openstack/charm-ceph-proxy/.tox/py3/lib/python3.6/site-packages/mock/mock.py", line 1330, in patched'
    b' return func(*args, **keywargs)'
    b' File "/home/alex/Projects/Canonical/git/opendev.org/openstack/charm-ceph-proxy/unit_tests/test_ceph_hooks.py", line 132, in test_client_relation_joined'
    b" hooks.client_relation_joined('client:1')"
    b' File "/home/alex/Projects/Canonical/git/opendev.org/openstack/charm-ceph-proxy/hooks/ceph_hooks.py", line 201, in client_relation_joined'
    b" data = {'key': ceph.get_named_key(service_name),"
    b' File "/home/alex/Projects/Canonical/git/opendev.org/openstack/charm-ceph-proxy/hooks/ceph.py", line 398, in get_named_key'
    b' ceph_user(),'
    b' File "/home/alex/Projects/Canonical/git/opendev.org/openstack/charm-ceph-proxy/hooks/ceph.py", line 52, in ceph_user'
    b' if get_version() > 1:'
    b' File "/home/alex/Projects/Canonical/git/opendev.org/openstack/charm-ceph-proxy/hooks/ceph.py", line 94, in get_version'
    b' error_out(e)'
    b' File "/home/alex/Projects/Canonical/git/opendev.org/openstack/charm-ceph-proxy/hooks/ceph.py", line 115, in error_out'
    b' sys.exit(1)'
    b'SystemExit: 1'
    b''