haproxy fails to start in sandbox when selinux is enabled on CentOS 7

[longshorej]

• Run the sandbox in CentOS 7.3 with SELinux enabled.
• After running sandbox run 2.1.1 -n 3 -f visualization, note that conduct info shows all bundles running.
• However, haproxy hasn't been started successfully - verify with docker logs and docker ps.
• After using setenforce 0 it starts up.

We need to either document this behavior and the SELinux-related config changes to whitelist the sandbox, or see if there's a way to mount volumes in Docker containers that is SELinux compatible.

Slack: https://typesafe.slack.com/conversation/C03DUBYSE/p1499373150822684