Ensemble base image must start Machine Agent

[Gustavo Niemeyer]

When the Provisioning Agent (bug #616504) detects that a new Machine State (bug #616487) was created in Zookeeper, it should start a new EC2 machine through the EC2 MachineProvider interface. This procedure must include the IP addresses for the Zookeeper cluster and the machine ID (bug #614495) so that the Machine Agent (bug #617042) can connect back and know its identity.

Given that scenario, the task described in this bug consists in the following:

The Ensemble base image being started must detect that the ensemble_zookeeper_hosts and ensemble_machine_id variables were made available, set the $ENSEMBLE_MACHINE_ID and $ENSEMBLE_ZOOKEEPER_HOSTS environment variables with the respective values, and fire the Ensemble Machine Agent at the end of the initialization logic.

The common logic for Ensemble Agents (described in bug #614508) which is used by the Machine Agent should be able to interpret these environment variables.