common_time: Move default election config to bcast.

Change the default master election and arbiration to use broadcast
instead of multicast.  As nice as the idea of using multicast for
discovery is, in the end it had just proven to be too unreliable in
random people's home infrastructures as well as the more complicated
managed infrastructures we are using internally.  Multicast still
works, and the service can still be configured to use it, but for now
we are switching to broadcast.

Also, add runtime checks to filter out our own broadcast traffic as
there does not seem to be any good sockopt in linux to do this for us
(there is one for multicast, but not broadcast).

Change-Id: I8ada3541cceca2e76c7a0c1a624a72026122c312