libqa: Switch to LVM base by default, 16GB disk size

Since the Docker DM default is 10GB volumes, we want at least enough
disk space to fill up one.  This is just preparatory work for further
commits.
Change disk layout to split storage between OS and Docker

This involves setting up LVM block devices for Docker data and
metadata.  Further, we generate a config file that tells Docker to use
that storage.

See https://bugzilla.redhat.com/show_bug.cgi?id=1144206