Simplify threading code in the gce_gahp. #3952

Remove unneeded code from when there were many fine-grained locks.
Improve cancelation code for gce jobs in the gridmanager. #3952
Modify the syntax for GCE metadata to be less Condor-specific. #3952

The gahp shouldn't have to understand Condor's goofy syntax for
specifying environment variables.