JCLOUDS-192. Move GCE to API v1beta15 - no new APIs added other than Zone/GlobalOperations, which are needed for zone-scoping changes.

Conflicts:
	google-compute-engine/pom.xml
	google-compute-engine/src/main/java/org/jclouds/googlecomputeengine/compute/functions/MachineTypeToHardware.java
JCLOUDS-192 - found a problem with paging list results for zone-scoped resources on 1.7.0, fixed it here too.