improve handling of ds instances during uninstall
    
    Ticket #2502
    
    * remove the "running" flag from backup_state in cainstance.py and
      dsinstance.py because it does not provide the correct
      information. In cainstance the running flag was never referenced
      because restarting dirsrv instances occurs later in dsinstance. In
      dsinstance when the running flag is set it incorrectly identifed the
      PKI ds instance configured earlier by cainstance. The intent was to
      determine if there were any ds instances other than those owned by
      IPA which will need to be restarted upon uninstall. Clearly the PKI
      ds instance does not qualify. We were generating a traceback when at
      the conclusion of dsinstance.uninstall we tried to start the
      remaining ds instances as indicated by the running flag, but there
      were none to restart (because the running flag had been set as a
      consequence of the PKI ds instance).
    
    * We only want to restart ds instances if there are other ds instances
      besides those owned by IPA. We shouldn't be stopping all ds
      instances either, but that's going to be covered by another
      ticket. The fix for restarting other ds instances at the end of
      uninstall is to check and see if there are other ds instances
      remaining after we've removed ours, if so we restart them. Also it's
      irrelevant if those ds instances were not present when we installed,
      it only matters if they exist after we restore things during
      uninstall. If they are present we have to start them back up because
      we shut them down during uninstall.
    
    * Add new function get_ds_instances() which returns a list of existing
      ds instances.
    
    * fixed error messages that incorrectly stated it "failed to restart"
      a ds instance when it should be "failed to create".