Teach DHCP Agent about DVR router interfaces

When DVR is enabled and enable_isolated_metadata=True,
the DHCP agent should only inject a metadata host route
when there is no port with the gateway IP address configured
on the subnet.  Add a check for DEVICE_OWNER_DVR_INTERFACE
when we look at each port's device_owner field, otherwise
it will always add this route to the opts file when DVR
is enabled.

Change-Id: I3ff3bb85105b8215b36535983016d8c0ff3d8cb7
Closes-bug: #1377307
Forbid update of HA property of routers

While the HA property is update-able, and resulting router-get
invocations suggest that the router is HA, the migration
itself fails on the agent. This is deceiving and confusing
and should be blocked until the migration itself is fixed
in a future patch.

Change-Id: I4171ab481e3943e0110bd9a300d965bbebe44871
Related-Bug: #1365426
Closes-Bug: #1378525
NSX: drop support to deprecated dist-router extension

The NSX plugin originally implemented its own 'dist-router' extension.
During the Juno timeframe the DVR extension was introduced and the NSX
plugin was ported to support both. At the same time 'dist-router' was
marked for removal in Kilo.

Now that Kilo opened, we can drop the deprecated one.

Change-Id: I7d6d2524ef6d5cb94a9e8514425a1a288eea7210
Merge "Teach DHCP Agent about DVR router interfaces"
Merge "NSX: drop support to deprecated dist-router extension"
Merge "Forbid update of HA property of routers"
Merge branch 'master' of https://github.com/openstack/quantum