Mock AWS credentials

[lhufnagel]

This should allow Moto-users to keep using existing tests without having to export dummy credentials from v1.3.7 onward. See also #1924

[coveralls]

Coverage increased (+0.0008%) to 92.496% when pulling cf5bd76 on lhufnagel:master into ed861ec on spulec:master.

[jakul]

@spulec Can this PR be merged?

[kgutwin]

While this isn't at all my use case, wouldn't this change affect users who mix-and-match Moto mocks and real AWS calls in the same test run? I know it uses os.environ.setdefault to avoid overwriting environment variables, but this would disrupt any case when valid credentials are present in ~/.aws or instance metadata.

[spulec]

Thanks!

[rjaduthie]

This will help simplify our work with moto and test build configuration.

Now the has been merged, will this be in the next release?.. and when might that be? Anything that I can help with?

[yan12125]

Theses changes appear to break a test suite written by another developer: #2058

[s0undt3ch]

Theses changes appear to break a test suite written by another developer: #2058

Same here.
Don't make these changes globally. If its for tests, do it in the tests...