bs_rdb: Add virsecretuuid && cephx_key

Allow passing either a libvirt secret UUID, or a cephx_key to --bsopts. Options are
conflicting, so error if both options given. This allows one to do this:

--bsopts="conf=/etc/ceph/ceph.conf;id=cephx_user;virsecretuuid=$MY_LIBVIRT_SECRET_UUID"
-OR-
--bsopts="conf=/etc/ceph/ceph.conf;id=cephx_user;cephx_key=$MY_KEY"

Signed-off-by: Scott Sullivan <ssullivan@liquidweb.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>