sheep: save updated config after version migration

Current config migration mechanism doesn't save updated version number
even after successful migration. This patch fixes the problem.

Related issue:
https://bugs.launchpad.net/sheepdog-project/+bug/1327040

Cc: Ruoyu <liangry@ucweb.com>
Cc: Giovanni Bellac <giovannib1979@ymail.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
configure: correct a variable name for configure option

Current configure script for sheepdog cannot handle build time
prameters (e.g. "--enable-zookeeper") well. This patch fixes the
problem.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>