When getting credentials from env, os_user_domain_name might be missed

[Jamstah]

The OS docs say that depending on the auth type, a different domain name might be needed, either OS_USER_DOMAIN_NAME, OS_PROJECT_DOMAIN_NAME, or OS_DOMAIN_NAME work as env variables.

http://docs.openstack.org/developer/python-openstackclient/authentication.html

My openstack install provides me with OS_USER_DOMAIN_NAME in its generated rc, so the credential finding here fails.

I think we should at a minimum add all options to CredEnvDomainName in identity.go:
OS_DOMAIN_NAME
OS_USER_DOMAIN_NAME
OS_PROJECT_DOMAIN_NAME
OS_DEFAULT_DOMAIN

Really, we should be picking the right one based on the credential type.