[OVN] Use STACK_USER's actual primary group in starting services
[OVN] Use STACK_USER's actual primary group in starting services

devstack/lib/ovn_agent previously hardcoded the expectation that there
is a group with the same name as $STACK_USER. This group exists in a
default Ubuntu installation, but not in all possible systems.

This change starts all services previously using this group under
$STACK_USER's actual primary group.

Change-Id: I2015157e07f8284199cd4926d7d83bc1bb637339

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