l3 agent network_device_mtu deprecation fix
l3 agent network_device_mtu deprecation fix

If the neutron l3 agent network_device_mtu if it isn't set there
is no need to log this warning message.

Change-Id: I7bdbad512bd7921bbcf0367d1bc2700d35585cc0

rdo-puppet-modules/puppet-neutron,redhat-cip/puppet-neutron,fuel-infra/puppet-neutron,IPnett/puppet-neutron,NeCTAR-RC/puppet-neutron,NeCTAR-RC/puppet-neutron,cloudbuy/puppet-neutron,aldavud/puppet-neutron,IPnett/puppet-neutron,plumgrid/puppet-neutron,NeCTAR-RC/puppet-neutron,cloudbuy/puppet-neutron,fuel-infra/puppet-neutron,redhat-cip/puppet-neutron,openstack/puppet-neutron,aldavud/puppet-neutron,aldavud/puppet-neutron,CiscoSystems/puppet-neutron,rdo-puppet-modules/puppet-neutron,openstack/puppet-neutron,CiscoSystems/puppet-neutron,redhat-cip/puppet-neutron,IPnett/puppet-neutron,plumgrid/puppet-neutron,CiscoSystems/puppet-neutron,cloudbuy/puppet-neutron,fuel-infra/puppet-neutron