Update the Containerfile https://github.com/umohnani8/machine-config-operator/blob/master/pkg/controller/build/assets/Dockerfile.on-cluster-build-template to be able to parse and install any extensions defined by the MachineConfig.

Grab the extensions from the extensions image in the `AS extensions` stanza. Then install it via rpm-ostree in the `AS configs` stanza.

Link to some existing code: https://github.com/openshift/machine-config-operator/blob/master/pkg/daemon/update.go#L2816-L2831 

Done when:

• Extensions from the MachineConfig is parsed and installed via the Containerfile

Card created from spike https://issues.redhat.com/browse/MCO-1175