User to be able to override endpoint

[gildub]

When a service endpoint URL is wrong in the catalog the user should be able to override it.

This is actually the recommended way by Openstack API Working Group guidelines:
http://specs.openstack.org/openstack/api-wg/guidelines/consuming-catalog.html

Historically because of the evolution of Openstack API versions has forced us to discover the version and force the base path. Although, at time, it helps word around a specific issue, it creates more issues down the road. This is why is should be avoided (and is not recommended/forbidden by guidelines) and makes the endpoint-override option even more relevant.