LBaaS: remove orphan haproxy instances on agent start

This change adds remove_orphans() handling to the haproxy
namespace driver. remove_orphans() is already called by
lbaas agent on start for all drivers

Closes-Bug: #1262885
Change-Id: I5deae8e56c2cd2deb1667e9646633fd59a94b34e
Merge "LBaaS: remove orphan haproxy instances on agent start"