Discovery URL querying functions

The existing discovery mechanism is geared around allowing users to
create appropriate client objects. It does not provide any means to
query version responses to determine the URLs of available versions.
This patch adds url_for (similar to the functions available on service
catalog) and data_for functions that can do lower level queries for
URLs and version response data.

Because of the client creation functionality the existing discover.py
cannot be imported from within keystoneclient due to cyclical dependency
resolution. The querying functions are therefore added to a private
keystoneclient file and exposed to the original Discover object via
inheritance. This private object can then be used within the
keystoneclient library.

Blueprint: endpoint-version-query
Change-Id: I7e1891536289c57ad3fbe8ef1c6e50b7344d83d5
Merge "Discovery URL querying functions"