Prevent races in opportunistic db test cases

Previously opportunistic db test cases used to share openstack_citest
database among all test cases, which could be run concurrently in
different test running processes.

With recent changes made to our CI, we now can create and drop
database schemas on demand in tests. Providing each opportunistic db
test case with its own DB will effectively prevent possible races.

Related-Bug: #1288916

Change-Id: I7f6e272eaeb776b6a645ba502853892e79312afd
Merge "Prevent races in opportunistic db test cases"