Add option to generate container image using ARG and ONBUILD

[ivotron]

Having builders generate images from a host's folder is useful for dealing with fast iterative dev cycles. However we'd like to be able to generate self-contained images that can be automatically built (e.g. put in dockerhub as automated builds). This can be achieved by using ARG on Dockerfiles so we can decide whether to build everything at image build time, or to defer to building at the enrypoint stage

[ivotron]

The ONBUILD option allows to defer the compilation to whenever someone is doing FROM cephbuilder:jewel and the ARG option allows to specify that we want to disable GENERATE_DOCKER_IMAGE. Doing this also requires pulling dependencies and entrypoint for ceph/base and ceph/daemon so that we can interchange a ceph/daemon by a customized image.