Fix volume metadata validation of identical to request

This change is for fixing bug 1264418 tempest cases.
During the creating volume process, driver can add specified
data to "metadata", but some tempest cases avoid to do so.
Use assertDictContainsSubset instead of assertEqual to check
the metadata. Also verified this changeset works.

Modified per Giulio's comment:
Use testtool.matcher#ContainsAll to assert if the dict contains
the sub dict.

Change-Id: Ief92b60d39acbde97f620b1a9ab943a13022d839
Partial-Bug: 1264418
Merge "Fix volume metadata validation of identical to request"