This change adds audit filter in paste ini file which can be used to add
this filter in existing request processing pipeline. Added new pipeline
with audit filter after keystone_authtoken filter.
Adding documentation about how to use this.
This change provides auditing on REST API side. Worker side changes will
be done later in another review and are not dependent on this.
Change-Id: I308796f5804aa55bc3d6496ded0504b469f00301
Partially-Implements: blueprint audit-cadf-events