multisite/test: running the multisite test script with custom devices

multisite/test: running the multisite test script with custom devices

following environment variables should be set to allow that:

DEV_LIST: comma separated list of devices that should be used by each
clusteri as bluestore block devices.
for example in: DEV_LIST=/dev/nvme7n1,/dev/nvme6n1
the device /dev/nvme7n1 to be used by cluster "c1, while /dev/nvme6n1
will be used by cluster "c2".

DB_DEV_LIST: similar mechanism for DB
WAL_DEV_LIST: similar mechanism for WAL

note that if any of these variables is missing, or has less values than
clusters, the default device would be used.

Signed-off-by: Yuval Lifshitz <ylifshit@redhat.com>