feat(neutron): split UnderstackDriver into L2 and Undersync mechanism drivers #1096

Job	Run time
ironic / clean	5s
openstack (octavia) / clean	4s
openstack (keystone) / clean	3s
openstack (placement) / clean	3s
openstack (skyline) / clean	5s
openstack (openstack-client) / clean	3s
openstack (horizon) / clean	4s
openstack (neutron) / clean	6s
openstack (nova) / clean	3s
ironic / build	-1s
openstack (glance) / clean	3s
openstack (cinder) / clean	5s
openstack (octavia) / build	-1s
openstack (cinder) / build	0s
openstack (neutron) / build	0s
openstack (nova) / build	0s
openstack (openstack-client) / build	0s
openstack (placement) / build	0s
openstack (horizon) / build	0s
openstack (keystone) / build	0s
openstack (glance) / build	0s
openstack (skyline) / build	0s
	42s

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