Clarify admin_delete_server_of_others test case

create_test_server() doesn't wait for server 'ACTIVE' by default now.
But this test case verifies 'Can a admin delete servers of others?'. So
we should wait until 'ACTIVE' here.

Change-Id: Ibb9e65166035c686c0b222033cb08f32a63f8b23
Merge "Clarify admin_delete_server_of_others test case"