recovery: check node before try to connect to

This check exist in the old code but was accidentally refactored out by some
ghost commit.

Check node validity before connecting will save a lot of unnecessary network
traffic during recovery. Let's summon it back.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
sheepdog stable release v0.6.2-rc1

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>