dog: enable 'cluster format -c x:y'

This patch enables 'cluster format -c x:y' option for users to set a
cluster-wide default scheme.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
tests/func: add a test dedicated for 'cluster format'

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>