Changed migrate_centralized_floating_ip() to return a
status in DvrLocalRouter class. Also changed the parent
method in RouterInfo class to return FLOATINGIP_STATUS_NOCHANGE,
which will cause the agent to not send an updated status for
the floating IP.
Also changed floating_ip_added_dist() to not use an
intermediate variable and just return directly.
Change-Id: I7dc4934308da95cf00a36b4ef1020aac7cef7d99
Closes-bug: #1816874