Change $JUJU_REPOSITORY directory

[mbruzek]

jujubox sets the $JUJU_REPOSITORY to /home/ubuntu . Charmbox is composed from jujubox and has the option of mounting the $JUJU_REPOSITORY directory for using charms on the host system. This mount will obfuscate the .juju directory on the container.

I would suggest (and hopefully make a pull requests) to change the JUJU_REPOSITORY directory to /home/ubuntu/charms to fix this issue.

Thanks.