Don't enforce project ID validation when unmarshallig log entries

[tseaver]

The back-end apparently returns numeric project IDs in some cases.

Update gcloud._helpers._name_from_project_path to allow passing its project argument as None to disable the validation.

Closes: #1806.

[dhermes]

The decision to fully disable the check instead of to make it optional was due to the deep nesting of method calls? Could we put a flag on Client to disable "globally"?

/cc @waprin

[tseaver]

@dhermes The deep nesting is part of it: the other part is that the validation was a carryover from other places where having the project match up was essential to allow re-use of the existing client, which isn't the case at all for log entries (there are no API calls which originate from them).

If the backend had a "list loggers" API (which it inexplicably doesn't), then we might need to be validating the project when unmarshalling its results.

[tseaver]

I guess somebody could call client.list_entries() and then try to navigate from one of the returned entries to its logger attribute and use the (potentially invalid) logger to make API calls. Seems a stretch, though.

[dhermes]

Sorry for going AWOL. 5 days, eek! LGTM. We can add in more hooks as they come up.

[waprin]

Btw, returning project numbers been acknowledged as a bug but apparently it won't get fixed for a few months since it will be fixed by some stack migration anyway.