reconfigurations of decision engine cause short lived google credentials not to be renewed

[StevenCTimm]

I observed the following:

1. I reconfigured the decision engine with a number of glideins to google cloud active at the time.
2. as a result the GceAuthFile string changed to a different hash, but the already-submitted glideins
   did not change the GceAuthFile
3. I observed that the old cached credential was no longer getting renewed, thus after a while causing
   the google cloud glideins to hold.. although apparently only at completion. i.e. the credential that
   the GceGahp uses to poll the cloud is OK, but the individual GceAuthFile used to submit and clean up afterwards is not.

This is something that in theory can happen with any credential but is much more likely to be visible
with Google credentials because they expire after 1/2 hour. We have seen NERSC jobs be caught out
with an old ssh key pair before too.