Wait for Keystone start after master node reset

Use protected Nailgun API URL to check that Keystone
is up and works fine before running tests.

Change-Id: I28a84f8e2a304e18cf54eb7d08b24f0ef67503f5
Closes-bug: #1401692
(cherry picked from commit 4a00e4565b879f110ff56a3c941c0bd81efa7086)
Merge "Wait for Keystone start after master node reset" into stable/5.1