provision/juju: starting to port instanceHealer from tsuru-healer

Related to globocom/tsuru-healer#3.
provision/juju: add lb field to elbInstance

This field is the name of the load balancer.
provision/juju: using fake provisioner in some tests

This does not make that sense, but it's necessary.

Related to globocom/tsuru-healer#3.
provision/juju: import instanceHealer from tsuru-healer

Related to globocom/tsuru#3.
provision/juju: register ELBInstanceHealer

Related to globocom/tsuru-healer#3.