Add test for showing volume image metadata

This patch adds test for the missing API (volume v2):
show image metadata for a volume.

Including:

[1] Add show volume image metadata API to v2 volumes_client
[2] Add unit test for the API
[3] Modify test case: test_update_image_metadata
[4] Add release note

Change-Id: Id428c64794085ffbcda5d500bb4996e3d8da15ee
4 files changed