Refactor some l3_dvr_db code
Refactor some l3_dvr_db code

check_for_fip_and_create_agent_gw_port_on_host_if_not_exists()
and create_fip_agent_gw_port_if_not_exists() were logging
messages that didn't always make sense, cleaned it up.

Also removed some duplicated code that set the port MTU.

Trivialfix

Change-Id: Iea82cb9f5efb76599ed312fa960aea953e062588

mahak/neutron,openstack/neutron,openstack/neutron,openstack/neutron,mahak/neutron,mahak/neutron