Adds help in keystone_authtoken config opts

The keystone_authtoken config options defined in
keystoneclient/middleware/auth_token.py didn't have help strings.
Without help, we couldn't generate documented config files via
introspection.

Fixes Bug1159039

Change-Id: I6d805432edf65db8161d6a6f4916185c4df6bb90
auth_uri (public ep) should not default to auth_* values (admin ep)

Fixes bug 1207517 by logging a warning when auth_uri (which should point
to the public identity endpoint) falls back on auth_* values (which
should point to the admin identity endpoint).

Change-Id: I2b051ae10197206f6954672f22e5bff32e3f6c2a