Signed-off-by: Tim Jones <me@prototim.com>

https://github.com/saltstack/salt/issues/2142

This has been causing issues for many people, as the remove_groups options is undocumented.  In the 2014.7 release this is changing, and remove_groups will default to false:
https://github.com/saltstack/salt/issues/13276

I'm going with false by default, as it's our use case and it will soon be the default.  If people believe this module should default to true and remove groups not listed, I think that's open for discussion, but we should at least add the option.

This also made it necessary to introduce an additional entry 'shell' into the
users lookup table as the formula previously conflated the shell used for
running the visudo command and the default shell to be used for user accounts.

Fixes: #48

Since we are using bash specific features and sometimes you can end up getting /bin/sh - see https://github.com/saltstack-formulas/users-formula/pull/30#issuecomment-44224106

Fixes #22

Since ebe5198f, if a user's pillar dict didn't contain sudo_rules, a broken
file.append state would be rendered (since some text is required). With
this patch, the file is still created/managed by the previous state, but
will be empty by default if created fresh. This seems a more sensible
default than assuming a default sudoer policy.

Further, since the first word on each rule line should be the user's
name, that is now assumed.

if you set an alternate prime_group this would break, replaced {{ name
}} with {{ user_group }}

Changed Private keys to have content within pillar rather than the salt
file repository.

Changes sudoers entry to get values from pillar rather than assuming
all sudo users want root.

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.

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

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

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