Fix Undefined jinja variable error with Salt 0.17.1

Ran into an error with Salt version 0.17.1:
Rendering SLS users failed, render error: Undefined jinja variable; line 32

After finding https://github.com/saltstack/salt/issues/8245, and working on
line 32, I surmised that line 28 might be the actual cause of the issue.

I just added a quick check to see if uid exists in user, just as the user uid
line does.

better sudoers support & default gid

add support for sudouser being False.
change to adding sudoers config to /etc/sudoers.d/<user>
adding the removal of /etc/sudoers.d/<user> on user removal or switching to sudouser being removed or set to false

Fixed user default shell source

Pulling 'shell' from pillar globally makes no sense, probably a mistake; pulling it from user instead.

Quick example of absent users

Fix requisites for user state

group requisites were wrong. They required {{ group }}_group, while {{ group }} was the actually added group.

Restructure according to new guidelines.