With this patch the allow_access and deny_access methods of
glusterfs_native will keep preexisting common names in the
affected GlusterFS volume 'auth.ssl-allow' option.
Also check in _setup_gluster_vol if 'auth.ssl-allow' is
set to a non-empty value to avoid semantically problematic
(and from Manila POV, useless) edge cases.
Change-Id: I952049d694509a338c7f56b45c5ef0872c3e7d70
Closes-Bug: #1439198