Improve command to check if nailgun API is ready

Docker containers start asynchronously
and each container may take some extra
time to deploy, but puppet could finish
sooner. Added to bootstrap_admin_node.sh
a better solution to wait for nailgun
API to be ready.

Change-Id: Ib4bac3dcb386b594db6f86faab226e1eba8951fb
Closes-Bug: #1314522