Add wait for connection to restore after l3 agent migration

- After l3 agent rescheduling connectivity is restoring for some
period of time so we need to wait a few minutes before checking
connectivity on instance

Change-Id: Iafbf677bb902d64ae7e33346f2701900e30c9342
Closes-Bug: #1401819
Merge "Add wait for connection to restore after l3 agent migration" into stable/6.0