fix postgres service name
fix postgres service name

in fedora postgresql is the service name and postgresql-server is
the package.[1]

os: Fedora release 23 (Twenty Three)
psql: psql (PostgreSQL) 9.4.5

i'm not entirely sure when this changed, but it's devstack is broken
in above environment.

[1]https://fedoraproject.org/wiki/PostgreSQL

Change-Id: Id940fed2a777ca469ce77402e1136251ba572359

Mahito/devstack,bigswitch/devstack,richliu/devstack,group-policy/devstack,richliu/devstack,dtroyer/devstack,noironetworks/devstack,openstack-dev/devstack,openstack-dev/devstack,group-policy/devstack,Mahito/devstack,samgoon/devstack,noironetworks/devstack,dirkmueller/devstack,dirkmueller/devstack,bigswitch/devstack,samgoon/devstack,bswartz/devstack,nawawi/openstack,nawawi/openstack,bswartz/devstack,dtroyer/devstack