tests/functional: use kill_zk_session to _kill_sheep with zk

- fix kill_zk_session.c that if ZNONODE returns, we simply return success

In this way we don't blind sleep for _kill_sheep with zk driver. This fixes
some false failure of tests with SSD drive and also boost tests a little bit

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
farm: don't check of the existing object before slice_write

Currently, we stores sliced objects for the farm store, so
sha1_file_exist() fails always.

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