bootstrap must use custom image for fast iterations

[Gustavo Niemeyer]

The bootstrapping is set up to create a new ensemble-ready image from the ground up through cloud-init. Besides being a bit buggy, the process of setting up everything takes some time. We want development iterations to be as fast as possible, and the bootstrapping process to be stable. A pre-built image will offer us that, so we must change the EC2 provider code to work in those terms, and create a reproducible means for setting the pre-built image up.

Once the basics of this work are done, bug #614490 should be addressed so that Zookeeper listens in the internal EC2 interface. Also, bug #614508 refers to a base class for agents, which has to be kept in mind since we'll be firing the machine agent inside that base image.