common/rc: assign SCRATCH_DEV_POOL to an array

common/rc: assign SCRATCH_DEV_POOL to an array

Many test cases use local variables to manage the names of each device in
SCRATCH_DEV_POOL. Let _scratch_dev_pool_get set an array, SCRATCH_DEV_NAME,
for it.

Usage:

	_scratch_dev_pool_get <n>

	# device names are in the array SCRATCH_DEV_NAME.
	${SCRATCH_DEV_NAME[0]} ${SCRATCH_DEV_NAME[1]} ...

	_scratch_dev_pool_put

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>