Look in /etc/concerto/medium_path for medium path

This avoids hard-coding implementation details of the live image
into bandshell. The live image can write the /etc/concerto/medium_path
file as part of its boot process, and we will pick it up.
fix bug in previous commit

The mountpoint directory should never be nil. If it is nil when the medium
is read-only, we will not be able to store static configuration on a read-only
medium.