update MAINTAINERS

I'm not working for NTT lab now.

Also, let's add Hitoshi to the master branch maintainers.  Unfortunately, I
have no time for sheepdog development currently.  He is the maintainer of the
stable branch and will do a good job instead of me.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@gmail.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>