Skip read-only test for CIFS

When using the CIFS protocol and trying to share a share by IP, the
share fails to instantiate and the manila service produces this error:

  InvalidShareAccessLevel: Invalid or unsupported share access level: ro.

There are no Manila Share backends that support CIFS shares with read
only access when authenticating with IP addresses[1]. This patch causes
the test to be skipped in such a case.

[1] https://docs.openstack.org/manila/latest/admin/share_back_ends_feature_support_mapping.html#mapping-of-share-drivers-and-share-access-rules-support

Change-Id: Ic30eaf3c2b7f75eb4bc62159faceb6bd30d7e03b
1 file changed