manage WSGI configuration

This patch improves the WSGI support:
 - introduce a wsgi_user and wsgi_group to avoid confusion with
   apache_user and apache_group
 - check /etc/httpd/conf.d/openstack-dashboard.conf is here and properly
   configured
 - ensure the log directory is owned by the WSGI user
 - ability to define the number of WSGI process/thread
 - test coverage and some refactoring of
   spec/classes/horizon_init_spec.rb

Closes-bug: #1265169

Change-Id: I39b52b2bc51ba04d59de81dc45b5a4ad82d0dc2c