Use OS_AUTH_TOKEN environment variable if possible

[larsbutler]

Instead of authenticating with username and password (and getting an auth token) for every request, we should try to use the OS_AUTH_TOKEN variable if it is set.

(python-swiftclient uses this env var, so adding this feature to zpm would be consistent behavior.)

[larsbutler]

We might also consider the same for OS_STORAGE_URL.