pluggable token auth driver

implements a means to plug-in authentication token drivers by means of
providing a token driver interface with a default (base) implementation
for SLAPI. this change remains backwards compatible, but also permits an
easy way to plug-in a different driver. token auth drivers are
responsible for authenticating with an identity provider and
building/returning a well formed jumpgate auth token. this is part of
the keystone identity/auth workflow.

implements: https://github.com/softlayer/jumpgate/issues/47