Add waiter to test_manage_snapshot_rbac to fix data race

This PS adds a waiter after attempting to manage a snapshot
in test_manage_snapshot_rbac to avoid data race conditions
causing the test to fail during resource_cleanup [0].

This also adds a clean up to the test which is currently missing.

[0] http://logs.openstack.org/77/537977/1/check/patrole-admin/c4e6884/job-output.txt.gz#_2018-02-01_02_47_26_276336

Change-Id: Ib62a6d7ebbc5af43278c97fedce3a863ecfd3575
1 file changed