EUCA-4915: Additional servicees for the LB VMs

The servo running inside the special, system-owned VM communicates with the Eucalyptus loadbalancer service front-end.
It retrieves the listener specification, register instances, and any additional parameters from the service, and changes the haproxy configuration accordingly.

This first commit contains the clients of the two extra service endpoints, specially created for the loadbalancer VMs.
'get_servo_load_balancers' uses DescribeLoadBalanacers api, with the instance ID set to the Marker parameter
'put_servo_states' will update the healtch check results for the instances and the cloudwatch metrics such as request count.