Remove `SKIP_PIP_INSTALL` feature

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
[BUGFIX] Define `zk_server_list` variable

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
[BUGFIX] Catch subprocess.CalledProcessError

For the `docker ps | grep zk_ambassador` command in the
`docker_python_helpers/run_storm_supervisor.py` script.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>