func/test: _kill_xxx methods use SIGTERM by default

Since the bug, sheep sometimes cannot be killed, is fixed by the commit
f473561, functional tests should use 'pkill' instead of 'pkill -9'
by default. Otherwise, some tests using zookeeper as their cluster
manager will be failure because zookeeper cannot receive the leaving
message at the first time.

For the purpose of killing program forcibly, _kill_xxx_force methods
are added. They kill programs with SIGKILL (-9).

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