github-bot: add IPs of Jenkins workers

Providing `$JENKINS_WORKER_IPS` environment variable at startup will
activate the whitelist implemented in the bot, validating who's allowed
to pushed Jenkins job updates as inline PR statuses on github.com.

Refs: nodejs/github-bot#142
PR-URL: #985
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>

setup/github-bot/resources/environment-file

@@ -9,3 +9,4 @@ LOGS_DIR=/home/{{server_user}}/logs
 JENKINS_API_CREDENTIALS={{envs.jenkins_api_credentials}}
 JENKINS_JOB_URL_CITGM={{envs.jenkins_job_url_citgm}}
 JENKINS_BUILD_TOKEN_CITGM={{envs.jenkins_build_token_citgm}}
+JENKINS_WORKER_IPS=147.75.69.113,147.75.73.189