Make quantum pipeline configurable from quantum.conf.

Make quantum pipeline configurable from quantum.conf.

Fixes bug 1029313.

The current api-paste.ini does not provide a way to choose a pipeline:
there is no way to switching a pipeline between keystone-enabled and
noauth pipelines without modifying the pipeline directly.

This commit introduces 'auth_strategy' flag to quantum.conf and
a pipeline used is determined depending on the flag. Supported values for
this flag are 'keystone' (default) and 'noauth' at the moment.

Change-Id: Ieafaf31eaaec2b02727ed5d3bd36c907e50aee5b