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).

daemon-common/common.py

@@ -679,7 +679,7 @@ def getNodes(zkhandler, node_limit, dom_uuid):
 
 
 #
-# via allocated memory
+# via provisioned memory
 #
 def findTargetNodeMem(zkhandler, node_limit, dom_uuid):
     most_provfree = 0