Add selector help and adjust flag name

1. Add documentation on the node selector flags. In the API, reference
the daemon configuration manual which now includes details in this
section; in the CLI, provide the help in "pvc vm define" in detail and
then reference that command's help in the other commands that use this
field.

2. Ensure the naming is consistent in the CLI, using the flag name
"--node-selector" everywhere (was "--selector" for "pvc vm" commands and
"--node-selector" for "pvc provisioner" commands).

api-daemon/pvcapid/flaskapi.py

@@ -1002,7 +1002,7 @@ class API_VM_Root(Resource):
                     type: string
                 node_selector:
                   type: string
-                  description: The selector used to determine candidate nodes during migration
+                  description: The selector used to determine candidate nodes during migration; see 'target_selector' in the node daemon configuration reference
                 node_autostart:
                   type: boolean
                   description: Whether to autostart the VM when its node returns to ready domain state
@@ -1297,7 +1297,7 @@ class API_VM_Root(Resource):
             name: selector
             type: string
             required: false
-            description: The selector used to determine candidate nodes during migration
+            description: The selector used to determine candidate nodes during migration; see 'target_selector' in the node daemon configuration reference
             default: none
             enum:
               - mem
@@ -1446,7 +1446,7 @@ class API_VM_Element(Resource):
             name: selector
             type: string
             required: false
-            description: The selector used to determine candidate nodes during migration
+            description: The selector used to determine candidate nodes during migration; see 'target_selector' in the node daemon configuration reference
             default: none
             enum:
               - mem
@@ -1629,7 +1629,7 @@ class API_VM_Metadata(Resource):
                     type: string
                 node_selector:
                   type: string
-                  description: The selector used to determine candidate nodes during migration
+                  description: The selector used to determine candidate nodes during migration; see 'target_selector' in the node daemon configuration reference
                 node_autostart:
                   type: string
                   description: Whether to autostart the VM when its node returns to ready domain state
@@ -1678,7 +1678,7 @@ class API_VM_Metadata(Resource):
             name: selector
             type: string
             required: false
-            description: The selector used to determine candidate nodes during migration
+            description: The selector used to determine candidate nodes during migration; see 'target_selector' in the node daemon configuration reference
             enum:
               - mem
               - memfree