Rename base_url,auth_base to endpoint_url,astakos

Refs: #9

This change affects both the kamaki.cli and kamaki.clients hierarchies
It is of semantic significance only
Backwards compatibility is preserved in kamaki.clients, so that
old code using base_url instead of endpoint_url will still work.
Make OutputFormatArgument.value a variable

The object was a property, although this served no purpose at all.