Guard dbus.start() with dbus.is_running()

Some platforms like Debian protect the dbus.service with
RefuseManualStart=True. "systemctl start dbus" fails with operation
refused (it is configured to refuse manual start/stop). On Fedora
"systemctl start dbus" is a no-op when dbus is already running.

Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

ipaserver/install/cainstance.py

@@ -981,7 +981,9 @@ class CAInstance(DogtagInstance):
         # cause files to have a new owner.
         self.restore_state("user_exists")
 
-        services.knownservices.dbus.start()
+        if not services.knownservices.dbus.is_running():
+            # some platforms protect dbus with RefuseManualStart=True
+            services.knownservices.dbus.start()
         cmonger = services.knownservices.certmonger
         cmonger.start()