[deployment][arvados-formula] selinux breaks many installation steps
Selinux enforces a series of restrictions on ports and paths that applications can bind or use.
In our setup, nginx needs to bind to many restricted ports, passenger needs to write into a restricted path (usually /usr/share/nginx/passenger_temp), etc.
All these fail when SELinux is enabled (ie, Centos-7 has selinux enabled by default).
The formula should:
- allow to manage the selinux rules required for Arvados
- allow to disable selinux
- we should document the changes so any admin that wants to manage some other way can do it.