Allow users to become sudoers automatically

[shuhaowu]

This PR introduces a sudoer option for each user, which will automatically add the user to the list of passwordless sudoers in the container. It is accomplished via files in /etc/sudoers.d, which should be supported in most distros afaik.

This should eliminate some unnecessary provisioning step and streamline the process.

r: @robvdl

[robvdl]

I like this but would personally have called the config option "sudo: true" rather than "sudoer: true".

I'm a bit late to the party to merge this though, two years have passed since.

Do you have an objection renaming this config option?

[mtausig]

This branch should be rebased to the current master, to check if it passes the tests.