Set postfix origin to correctly deliver mail for local users
(such as root's cron errors)
Set two variable files from site.yml

That way we can override all the config from a central location. The
`user.yml` file overrides `defaults.yml`. `user.yml` was added to
`.gitignore` in order to allow a user to change it without having to
worry about merges. It's easy to do a diff to see what changed between
the defaults and your own settings.

Needs documentation, this is just a proposal.

Refs #39
Remove variables from roles

Refs #39
Comment defaults

That way the default values will not be applied by mistake. The user
needs to uncomment the lines from `vars/users.yml` and set them.

Also renamed openvpn variables.

Refs #39
Add explanations to the variable files

Refs #39
Add extra comment to sections

That way it's more convenient to uncomment an entire paragraph.