Formalize deprecation of token_api.list_tokens

The token_api.list_tokens method should really only be used internally
to keystone.  This has been the case (as per the docstring) since the
Havana development cycle.  This patchset formalizes the deprecation and
slates the public access to token_api.list_tokens to be removed once
Icehouse ships.

bp: deprecated-as-of-icehouse
Change-Id: I30a066d3775e67fd4a84f07dc351fcfd62751045
Merge "Formalize deprecation of token_api.list_tokens"