Add support for LBaaS agent

This patch adds support for installing and configuring the LBaaS agent
on one or more hosts. At this time we only support haproxy. The default
value is empty list, in which case LBaaS will not be configured.

Change-Id: I172dce2b2d9b1841a9a8e8fe290b766674020c4e
Merge "Add support for LBaaS agent"