EUCA-4597: Haproxy interface in the VM
This is the initial implementation of the haproxy interface:
-- haproxy config file is updated given the listener speficiations
-- loadbalancer protocol, port, instance port is supported
-- NOT IMPLEMENTED: specifying instance protocol, instance health check, SSL
-- haproxy process is restarted when the config file is changed (reconfigure)
-- unit test is in haproxy_test.py