Added a Common KafkaConsumer for all of the plugins

 - Added the common consumer for all request messages
 - Updated the OpenStack plugins to use the common consumer
 - Included the access_credentials topic into this consumer
 - Added the json schemas for the openstack, vrops and cloudwatch
   access credentials
 - Updated the Openstack plugins to use access_credentials messages
 - Added and updated all necessary tests
 - Updated installation documentation

Change-Id: I1486cf7230e351e5dbf18464110e5780d392eeeb
Signed-off-by: Helena McGough <helena.mcgough@intel.com>