Support 3rd Party Elements for reddwarf mysql and reddwarf percona images

The reddwarf team needs to come up with a couple of new elements that can be used to bring up the reddwarf-mysql guest agent and the reddwarf-percona-mysql guest agent. We would like to do so by including the necessary elements in our code base and dynamically link them during diskimage-builder execution.

example: disk-image-create -o myimage -p /home/ubuntu/reddwarf/element vm reddwarf-guest mysql