Add wait until node became offline

In test HA restrat we destroy 2 nodes
and then run ostf. Nailgun see that nodes as online
and as result ostf tests failed on attempts
to ssh to offline nodes.
So add method wait() to verify that nodes became
offline in nailgun and then only run ostf

Change-Id: I8740e9305be380e897fb7f68d2ff4e86612914fd
Closes-Bug: #1327193
Merge "Add wait until node became offline"