Pacemaker support for OVN DB servers

Pacemaker support for OVN DB servers

This patch enables OVN DB servers to be started in master/slave
mode in the pacemaker cluster.

A virtual IP resource is created first and then the pacemaker OVN OCF
resource - "ovn:ovndb-servers" is created. The OVN OCF resource is
configured to be colocated with the vip resource. The ovn-controller and
Neutron OVN ML2 mechanism driver which depends on OVN DB servers will
always connect to the vip address on which the master OVN DB servers
listen on.

The OVN OCF resource itself takes care of (re)starting ovn-northd service
on the master node and we don't have to manage it.

When HA is enabled for OVN DB servers, haproxy does not configure the OVN DB
servers in its configuration.

This patch requires OVS 2.7 in the overcloud.

Co-authored:by: Numan Siddique <nusiddiq@redhat.com>
Change-Id: I9dc366002ef5919339961e5deebbf8aa815c73db
Partial-bug: #1670564