added JCloudsPropertiesFromEnv and JCloudsPropertiesFromEnvTest

* ignored CredentialFromEnv inside JcloudsResolver
* simplified JcloudsResolver.newLocationFromString
* added JcloudsPropertiesFromEnv to retrieve only the properties for jclouds

NB: only property with this format are considered:
* brooklyn.location.named.NAME.key
* brooklyn.jclouds.providerOrApi.key