Add L3 VRRP HA base classes

Add L3 HA base classes on the plugin side. A new admin-only ha
attribute is added to the API router resource. Conversion from
or to HA router is possible. Each tenant gets a single network
used for HA traffic. The tenant_id for that network is set to
'' so that it isn't visible via the CLI or GUI. A new table
is added to map a tenant to its HA network. Specific HA
attributes are added to the extra router attributes table.
Finally, each HA router gets a port on the HA network, per
l3 agent it is scheduled on. A new table is added to track
these bindings. A new table is added in order to track
VRID allocations.

DVR integration is not expected to work. Any issues will
be reported as bugs and handled after the feature merges.
Migrating a router to HA or from HA works server side
but is not expected to work (Yet) agent side. This will be
dealt with as a bug in the future.

DocImpact
Partially-implements: blueprint l3-high-availability
Change-Id: I9d935cf5e0c231e8cb7af5f61b9a9fc552c3521e
Co-Authored-By: Assaf Muller <amuller@redhat.com>
Merge "Add L3 VRRP HA base classes"