Always enable devstack service

When the plugin is enabled, we should always enable the service, there's
no benefit in having an extra invocation - and it can only confuse users
when they forget setting it up.

For more info see:
http://docs.openstack.org/developer/devstack/plugins.html

Change-Id: Ifcabc0955cb54f66388cac0c74b70c70ba65a1f9

devstack/README.md

@@ -8,8 +8,4 @@ For example:
 
     enable_plugin astara http://github.com/openstack/astara
 
-You will also need to enable the service:
-
-    enable_service astara
-
 For more info see: http://docs.openstack.org/developer/devstack/plugins.html

devstack/settings

@@ -1,3 +1,5 @@
+enable_service astara
+
 for svc in q-dhcp q-l3 q-fwaas q-lbaas q-meta q-vpn; do
     if is_service_enabled $svc; then
         echo "Disabling $svc in favor of Astara."