Feature: cloud LoadBalancers support

Gives basic support for cloud load balancers that do not require provisioning a VM (e.g. so can add support for AWS's ELB, CloudStack load balancer, etc).

Replaces https://github.com/brooklyncentral/brooklyn/pull/958, because ELB is being moved to a different repo so don't pull in the 10MB dependency on aws-sdk jar.