[bradm, r=gnuoy] Handle case of empty nagios_servicegroups setting

config.yaml

@@ -168,6 +168,7 @@ options:
   nagios_context:
     type: string
     default: "juju"
+    type: string
     description: |
       Used by the nrpe-external-master subordinate charm.
       A string that will be prepended to instance name to set the host name
@@ -175,3 +176,9 @@ options:
           juju-myservice-0
       If you're running multiple environments with the same services in them
       this allows you to differentiate between them.
+  nagios_servicegroups:
+    default: ""
+    type: string
+    description: |
+      A comma-separated list of nagios servicegroups.
+      If left empty, the nagios_context will be used as the servicegroup

metadata.yaml

@@ -22,4 +22,3 @@ provides:
   nrpe-external-master:
     interface: nrpe-external-master
     scope: container
-    gets: [nagios_hostname, nagios_host_context]