Add missing waiter to test_tagged_attachment

test_tagged_attachment was failing when Tempest was set to connect
via fixed IP. The test was trying to attach an interface to an
instance that was still in the building state. Adding a 'wait_until'
to the method creating said instance fixes the issue.

Change-Id: Ia1252a22c54e3a84a8e5e779ed6ea5ee593760da
Closes-Bug: #1860680
1 file changed