Remove MAGNETODB_CONF_DIR on devstack cleanup

Remove /etc/magnetodb (or what is set as
MAGNETODB_CONF_DIR) as a part of
devstack/cleanup.sh

Change-Id: I4e35f271f4a2c4f0c9222e44040b77acccc9ef4d
Merge branch 'remove-configs-on-cleanup' into bp/backup-repo