Explicitly enable the Centos 7 firewall

As usual, Digital Ocean boxes ship with insecure defaults, contrary to a
basic installation from an official ISO.
Reload OpenSSH server during build env setup

If you edited sshd_config the changes won't take effect until the next
reboot otherwise.
Add Centos 7 steps to release.md
Fix attribute lookup typo
Use `usermod -aG` to add www-data to gitlab-www

The `usermod -G` invocation changes the primary group ID of the user.
With `usermod -aG`, you add a supplementary group membership to the
user, preserving their primary group.
Update usermod command for passenger
Wrap lines in README.md
Add a note about SELinux and web servers
Merge branch 'centos7-do' into 'master'

Build / release documentation fixes for Centos 7

See merge request !197
Merge branch 'web_server_fixes' into 'master'

Web server fixes

See merge request !201
Document Debian 7 Upstart workaround