Add a new credential provider to use a list of creds

This commit adds a new credential provider to use credentials provided
from a list of available credentials.

Partially implements bp test-accounts
Change-Id: I69c3105f8b413102ea567ccb92965572ce03d679
Remove skipping flavor_access_add/remove related tests

At change Ibf741059c3a7a94543f1a2230fe7048692e1db0c, skip the
flavor_access_add/remove related tests for changing 'tenant' to
'project' in nova v3 REST API. But the v3 API plan is changed,
this kind of change will be introduce back when we have micro-version.
So remove the skip decorator for now.

Change-Id: I9ba1f0c10b7e976df303a9800bea2eea62f43891
Merge "Add a new credential provider to use a list of creds"
Merge "Remove skipping flavor_access_add/remove related tests"
Merge branch 'master' of https://github.com/openstack/tempest