Wait until Nailgun set suspended node as offline

When we run OSTF after suspending one of controllers
it tries to use proxy server on some of online nodes.
We should wait until Nailgun updates suspended node
status to 'offline' to avoid proxy connection errors.
Also run OSTF after each node shutdown.

Change-Id: I0d8287706fbc199cb49f3af2552b14c73fdc94cb
Closes-bug: #1352862
Merge "Wait until Nailgun set suspended node as offline" into stable/5.1