Fix main dashboard

The double-quotes in the main_prometheus dashboard do not need to be
escaped.  It just results in double-escaping in the actual Grafana
dashboard which breaks things:
  expr: avg(openstack_api_check_status{service=~\\"keystone.*public.*\\"})

Change-Id: Ia8ba02c081bf51b785dcd22a75040d540f00e4c3
1 file changed