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
diff --git a/lib/databases/postgresql b/lib/databases/postgresql
index 204c257..852bac4 100644
--- a/lib/databases/postgresql
+++ b/lib/databases/postgresql
@@ -102,7 +102,7 @@
     elif is_fedora || is_suse; then
         install_package postgresql-server
         if is_fedora; then
-            sudo systemctl enable postgresql-server
+            sudo systemctl enable postgresql
         fi
     else
         exit_distro_not_supported "postgresql installation"