Adding an implementation of GetZoneByProviderID & GetZoneByNodeName for
GCE.
This is related to ticket 50926.
This was tested as part of the ongoing separate GCE cloud provider work.
Added unit test.
Fix for wojtek-t (borrowed from FengyunPan)
Each major interface is now in its own file. Any package private
functions that are only referenced by a particular module was also moved
to the corresponding file. All common helper functions were moved to
gce_util.go.
This change is a pure movement of code; no semantic changes were made.
