Prevent L3 agent looping calls from hanging

This patch adopts several measures to prevent _sync_routers_task
and _rpc_loop from hanging because of subprocess.Popen.communicate
not returning.

1) Perform a sleep everytime a command is completed, similarly to
what is done in openstack.common.processutils.execute
2) Disable by default GARP, as kernel crashes caused by arping
have been observed
3) Prevent a non-critical keyerror in _router_removed from triggering
again a full sync, which might put the system under significant load.

This patch also adds debug log statements aimed at improving the
ability of debugging similar failures.

Change-Id: I003316bce0f38b7d2ea7d563b5a0a58676834398
Partial-Bug: 1224001
(cherry picked from commit 591ee00a67fbbe5f106ba12140b9f9420dee5907)