Always set the endpoint_url by defaulting to the public URL

[camporter]

If the endpoint_type cannot be determined, default to the public URL. Somewhat addresses #871 by avoiding an error if the user doesn't explicitly ask for public, private, or custom.

[coveralls]

Coverage increased (+0.007%) to 85.433% when pulling 75ad375 on camporter:always_set_endpoint_url into e17b21d on softlayer:master.