MT#55941 Add sites_enable to sites.yml

This will replace the geo_cluster.enable knob from config.yml, as it
needs to be a out-of-band option and not one from within the site
specific configuration YAML files.

Change-Id: I1c062091bb3bc1c7e52311c09524dabec5a00e97

schemas/sites.yml

@@ -2,6 +2,8 @@ name: sites.yml
 desc: Kwalify schema for NGCP sites.yml
 type: map
 mapping:
+  "sites_enable":   { type: str, required: yes, enum: [ yes, no ],
+                      desc: "Enable or disable multi-sites support." }
   "sites":
     type: map
     required: yes