Bump GCE API URL to `v1beta15`

This updates the base URL -- and only the base URL -- to reflect the current
API version.  While this will lead to breakage where the API structure has
changed, the fact that the previous `v1beta12` API no longer exists leads to
the same outcome in either case.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>
Update action docstring to match reality

The documented default zone did not match the in-code default value; this
updates the documentation to reflect that.  (It still sets the default way
down in the bowels of the code, rather than in-situ, to minimize code churn.)

Signed-off-by: Daniel Pittman <daniel@rimspace.net>