Refs: #3874

Use kamaki.clients.astakos.AstakosClient as a cached astakos client to get
user information and, most importantly, endpoints. Allow users to authenticate
multiple tokens on the same session. In every session there must be at most
one authentication per user/token.

Major change: top kamaki.cli.commands class now contains a base_auth field
with the cached authenticating client. All urls are drained from this field.