rpm: add --with-zbd option

rpm: add --with-zbd option

libzbd-devel is not available in any distros official repo at the time
of writing. but we packaged it for RHEL8/CentOS8. and uploaded its
packages to http://apt-mirror.front.sepia.ceph.com/lab-extras/8/. to
build test it to avoid the bitrot of ZBD bluestore backend at a minimal
level. but we cannot do more than this. because

- the kernel shipped by RHEL/CentOS8 does not have zoned block device
  support enabled. see https://zonedstorage.io/distributions/linux/
- and we don't have zoned block device for testing in our lab.

so, in this change, "--with-zbd" option is added so we can pull
in the libzbd-devel package on demand.

Signed-off-by: Kefu Chai <kchai@redhat.com>