vstart.sh: Allow to run multiple cluster instances.

This patch adds a few ENV variables, so you can use vstart.sh
multiple time to launch multiple clusters

CEPH_DIR -> The working directory of the cluster
CEPH_DEV_DIR -> the dev directory of the cluster
CEPH_OUT_DIR -> the output directory of the cluster
CEPH_RGW_PORT -> the default radosgw port to start with

All theses new variables are set to default values if not specified,
which ones does not change the previous behaviour of vstart.sh

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>