Docker Container Plugin

Adds a docker content plugin, monitoring for content of type "containerimage" in certificates and if found, copies that cert/key pair to /etc/docker/certs.d/ with filenames docker expects.

Certs should be kept in sync, if the entitlement is removed, the docker copy should go with it. This is implemented by looking for any other files that look like key/cert pairs and cleaning them up. i.e. we assume ownership of the cert pairs within this directory.

The hostname directory within certs.d is still unknown and will need to change. (currently redhat.com) We may also need to include a CA cert. These are the only outstanding changes I'm waiting to hear back on.