Fix bs_rbd.c slurp_to_semi() not consuming terminating semicolon

slurp_to_semi() would not consume the terminating semicolon on a
option specified in --bsopts, so any options other than the first
would be ignored as invalid.

Signed-off-by: Thomas Matysik <thomas@belton.co.nz>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Add "cluster" option to bs_rbd.c to specify cluster name

This patch allows the Ceph cluster name to be specified in --bsopts
using the 'cluster=' option.

Signed-off-by: Thomas Matysik <thomas@belton.co.nz>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>