Add test for showing volume metadata item

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

Including:

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

According defcore(interop) patch is:
Ica8b57f5f5392e2bf47e368e9b10ef9fa5695c30

Change-Id: Idb9d198383ce93453707f1204806e4ae82c1e46a
4 files changed