add JCloudsPropertiesFromEnv

Extends (and thus replaces) https://github.com/brooklyncentral/brooklyn/pull/773.

Controversial things:
* Adding the location-metadata.properties configuration file.
* Not accepting things like `brooklyn.jclouds.privateKeyFile` any more (need to specify it per-provider).
* Not accepting unqualified config names - but context of use for `JcloudsPropertiesFromBrooklynProperties` is JcloudsResolver so it's not called directly by users; good to not accept these unqualified config vals directly from brooklyn.properties in my opinion.