Allow customizing the GCE metadata service address via an env var.

[craigcitro]

The goal here is to make it possible for a user of a binary that depends on
this library (eg the google cloud SDK) to be able to customize where it looks
for the GCE metadata service. (An adventurous user can already customize the
GCE metadata service location via the existing global vars in this library.)

The only bit of awkwardness here is really the test: since this is a top-level
statement, reloading is the only way to ensure it works.

PTAL @jonparrott

(and why yes I DID just completely copy-paste the commit description from the oauth2client PR.)

[dhermes]

@craigcitro Did you want to merge in both libraries? (It's pretty unlikely oauth2client will ever get another release.)

[craigcitro]

@dhermes Yep, I purposely sent the PR for both.

@jonparrott mentioned the same about oauth2client, then remembered that he had to add something silly (update the classifiers?), so it was an opportune time for me to sneak it in. 😉

[craigcitro]

OK, updated to use the constants in environment_vars -- PTAL

[theacodes]

@craigcitro LGTM, but Travis is mad about you not using 4 spaces for indentation. :)

[craigcitro]

As well he should be. Fixed.

(Do you guys autosquash or should I squash?)

[theacodes]

(Do you guys autosquash or should I squash?)

We autosquash

[theacodes]

Thanks, @craigcitro!

[dhermes]

🚗 squash