Fix images tests with tenant isolation disabled

This commit fixes the images clients for when tenant isolation is
disabled. The base image test class still had a reference to the old
auth style to get the tenant name if tenant isolation was disabled.
The recent auth_provider refactor didn't update this call so the test
would fail. This commit updates it to use the new credential dict to
get the tenant_name

Closes-Bug: #1277732

Change-Id: I42bace92e5f633d410c7a59299680caafff680f9
Merge "Fix images tests with tenant isolation disabled"