Wait also for keystone-2 backend for nova floating ips

Sometimes, keystone admin backend is not ready and this
leads to 504 errors during floating IP creation

This commit adds additional waiting exec.

Change-Id: Ia5d2b15b4b68b5692bb04103d62abc0f840232f0
Closes-bug: #1381982
Merge "Wait also for keystone-2 backend for nova floating ips" into stable/5.1