Python Application Default Credentials should checked for GCE environment last

[anthmgoogle]

The implementation of get_application_default has an issue that it detects the environement before fetching the credentials. However, the check for GCE can hang on certain networks for up to 30 seconds (See #93).
This should be changed to match the Java implementation where this is not queried until the end because of its side effects. This means that the can can only occur in a failure case.

To give a little more detail; the java code roughly says:

if (testForWellKnownFile(...)) {
return credsFromWellKnownFile();
} else if (testForGae(...)) {
return credsFromGae();
} else if (testForGce(...)) {
return credsFromGce();
}

on the flip side, the python code says:

if (well_known_file_exists()):
env = 'WELL_KNOWN_FILE'
elif (detect_gae()):
env = 'GAE'
elif (detect_gce()):
env = 'GCE'
...
return CredsFromEnv(env)

[theacodes]

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