fence_gce: add earlyexit parameter to power_cycle_instance

[megelatim]

If earlyexit is enabled then power_cycle will exit before the execution of the power cycle on the gce instance is complete.

[knet-ci-bot]

Can one of the admins verify this patch?

[oalbrigt]

ok to test

[megelatim]

Is there anything that I need to do for the test?

[oalbrigt]

No. I just left it open for a couple of days so other devs had a chance to comment.