Remove full stack log noise

Remove full stack log noise

"neutron-server isn't up yet" logs are useless because if
you time out when waiting for the server to start that information
will be in the trace. When you don't time out, the log is just spam.

Same reasoning for the "There are %d agents running!" log.

Also made the agents_count parameter mandatory for the
wait_until_env_is_up method because having a default of 0, or any
other default makes no sense. There's no reason to ever call that
method without specifying the agents_count. This method used
to be used with agents_count == 0 by the server to make sure
its up (And responding to REST calls), but the Neutron server
fixture now uses server_is_live method (Which calls list_networks)
instead.

Change-Id: Ifd6abd04ddaacc9976cb2a75269443f870b47c5b