ML2: Update help text for path_mtu
ML2: Update help text for path_mtu

Update help text to improve clarity regarding actual impact
of this option. Also remove ability to set < 0 because
negative MTUs do not exist and code only handles >= 0.

Co-Authored-By: Matt Kassawara <4c2e6149ef19dba98a7fe8873ce28b2098aceda7@gmail.com>
Change-Id: I810cc0a716643471fd1365420b1034a0bfe81cb9

sebrandon1/neutron,bigswitch/neutron,klmitch/neutron,huntxu/neutron,MaximNevrov/neutron,mahak/neutron,noironetworks/neutron,igor-toga/local-snat,mahak/neutron,mahak/neutron,igor-toga/local-snat,cloudbase/neutron,openstack/neutron,wolverineav/neutron,sebrandon1/neutron,huntxu/neutron,cloudbase/neutron,eayunstack/neutron,klmitch/neutron,noironetworks/neutron,MaximNevrov/neutron,openstack/neutron,eayunstack/neutron,bigswitch/neutron,dims/neutron,openstack/neutron,dims/neutron,wolverineav/neutron