Start ovs if ovn-northd is enabled
Start ovs if ovn-northd is enabled

There are usecases where ovn-northd is running standalone with ovs, the
suggested local.conf is among the sample files (see: [0]). The service
however doesn't start as it needs OVS to run. This patch makes OVS
installed and started by ovn devstack plugin if ovn-northd is enabled in
local.conf.

[0]: https://opendev.org/openstack/neutron/src/branch/master/devstack/ovn-db-local.conf.sample

Change-Id: Ib12a076e50f010464735c6779d0e67bb218b66be
Closes-Bug: #1871730

mahak/neutron,mahak/neutron,openstack/neutron,openstack/neutron,openstack/neutron,mahak/neutron