Spawn arping in thread to speed-up floating IP

Change _send_gratuitous_arp_packet() to spawn a thread to call
arping after a floating IP is assigned.  This way it doesn't
stall _process_routers() from returning quickly due to calling
pool.waitall().

Fixes Bug: 1233391

Change-Id: Id1f5eb75c222ba6a0935a294e3973292f50d0559
Merge "Spawn arping in thread to speed-up floating IP"
change assertEquals to assertEqual

According to http://docs.python.org/2/library/unittest.html
assertEquals is a deprecated alias of assertEqual.

Change-Id: I4030e9f9b184af60eb9253b3a1d7e1cc929d502d