Exclude volume tags while listing instance tags

When a volume is attached to an instance and then detached, a metadata
gets associated with the volume: 'readonly' = 'False', even if the volume
didn't have this entry before. The patch
https://review.openstack.org/#/c/70085/ against Nova adds support for
volume tags, and as a result, it breaks a test case where we are
asserting that no tags are left after deletion of them.
So this patch makes sure that we assert only for 'instance' tags, and
not volume tags.

Change-Id: Icdd8c3a2e6c9d3264afc141ad516ee06c6127336
Merge "Exclude volume tags while listing instance tags"