Skip service_resume if already enabled

Repeatedly calling service_resume for a service that
is already enabled can lead to unintended consequences.
This adds a check to only resume a serice if it is not
enabled.

Related-Bug: #2058505

charmhelpers/core/host.py

@@ -253,6 +253,11 @@ def service_resume(service_name, init_dir="/etc/init",
                      parameters to the init system's commandline. kwargs
                      are ignored for systemd enabled systems.
     """
+    if service('is-enabled', service_name):
+        log('service {} already enabled - skipping resume'.
+            format(service_name), level=INFO)
+        return
+
     upstart_file = os.path.join(init_dir, "{}.conf".format(service_name))
     sysv_file = os.path.join(initd_dir, service_name)
     if init_is_systemd(service_name=service_name):