DCM_API_VERSION can be confusing

[igable]

Today users must specify an api version via an environment variable DCM_API_VERSION=2014-08-03 however the URL of the endpoint also contains that string as per DCM_ENDPOINT="https://dcm.enstratius.com/api/enstratus/2015-05-25"

This redundancy has been repeatedly a source of confusion.

I propose that that if DCM_API_VERSION is not set that this is extracted from the DCM_ENDPOINT. If a string matching YYYY-MM-DD isn't found at the end end of the URL in this case we will throw an error.

thanks to @jimsander and @cbrandtw0rk for this feedback.