Don't require version to create Client instance

We currently require a version to always be passed to discover the
client version that should be loaded. However, this information is
commonly present in the URL instead. The current behavior forces
consumers of the library to keep the required version around and/or to
strip it themselves from the URL.

This patch relaxes that requirement by making the version a keyword and
requesting instead an endpoint to be passed. The patch gives priority to
the version in the endpoint and falls back to the keyword if the later is
not present.

Follow-up patches will improve this code making it interact a bit more
with the endpoint's catalog.

Closes-bug: #1395714
Change-Id: I4ada9e724ac4709429e502b5a006604ca0453f61