4138: general GCE fixes

4138: general GCE fixes

* JSON credential file
** GCE credentials are delivered as a JSON string (and the key is formatted as a multi-line RSA private key). Let the GCE config file specify a path to the JSON credential file for simplicity.
* Accept NodeSizes addressed by id or name
** In EC2, NodeSizes are identified by the 'id' field.  In GCE they are identified by the 'name' field.  Allow the Node Manager config module to accept either.