test_create_router_gateway_fails fixes
test_create_router_gateway_fails fixes

- Use the existing network.  Otherwise this test fails earlier than
  intended.  (with NetworkNotFound)

- Use a dedicated exception to avoid accepting other exceptions
  like NetworkNotFound

- Move the test for L3_NAT_dbonly_mixin into a separate class.
  Skip it if the plugin does not inherit L3_NAT_dbonly_mixin.

Change-Id: Ic5d5162320bf3ff1bb8b09de3e8200ca46023b32

apporc/neutron,mahak/neutron,openstack/neutron,eayunstack/neutron,dims/neutron,openstack/neutron,huntxu/neutron,glove747/liberty-neutron,wolverineav/neutron,igor-toga/local-snat,glove747/liberty-neutron,chitr/neutron,cloudbase/neutron,igor-toga/local-snat,wolverineav/neutron,sebrandon1/neutron,MaximNevrov/neutron,bigswitch/neutron,dims/neutron,eayunstack/neutron,klmitch/neutron,asgard-lab/neutron,sebrandon1/neutron,noironetworks/neutron,apporc/neutron,asgard-lab/neutron,yanheven/neutron,noironetworks/neutron,huntxu/neutron,yanheven/neutron,cloudbase/neutron,mahak/neutron,klmitch/neutron,MaximNevrov/neutron,bigswitch/neutron,openstack/neutron,jumpojoy/neutron,jumpojoy/neutron,mahak/neutron,chitr/neutron