This allows the user to run their own devstack inside the containers.
Yay!
Also, we handle file permissions cleanly: in docker-entrypoint.sh we
chmod the data and edx-platform files to the same UID of the user on the
host machine. No more permission headaches!
When running "wait-for-mysql.sh", the "echo 'show tables' | manage.py
dbshell" command is always considered a success, even if the database is
unreachable ($? == 0). So we replace this command by a global system
check.
This closes #3.
