Add support for encrypting backend Keystone HAProxy traffic

This patch introduces an optional backend encryption for Keystone
service. When used in conjunction with enabling TLS for service API
endpoints, network communcation will be encrypted end to end, from
client through HAProxy to the Keystone service.

Change-Id: I6351147ddaff8b2ae629179a9bc3bae2ebac9519
Partially-Implements: blueprint add-ssl-internal-network