* Add support for 'template' in 'user_files'

* Fix-up wrong nesting level for template value

* Small quality improvement for push upstream.

* Consistency improvement for variable name

This reverts commit 48416398e3.

Fixing my previous change which errors in a particular scenario.

Error: Conflicting ID 'users_ssh_auth_source_username_0' when keys are added and removed simultaneously.

Fixing the conflict and sending a new pull request

Fixes #118
Duplicate of #120

Fixes: 150

* add ability to configure prime_group without gid

If the user to be created is a system user, it makes no sense to create
him a primary group which is not a system group too.

If a primary group is set on the user, and a authorized_keys is provied in ssh_auth_file, the formula fails. This solves that by using the user_group set earlier in the formula

Unreasonable values for 'expire' (after 9999-12-31
on Linux, before 1975-01-01 on *BSD) get divided
by 86400 (number of seconds in a day) when too big
or multiplied by 86400 when too small.

Tested on CentOS 6 (Salt 2015.5.5) and FreeBSD 10.2
(Salt 2015.8.0) with following values:

  - 24854 (2038-01-18 in days since epoch)
  - 157766400 (1975-01-01 00:00:00 UTC in seconds since epoch)
  - 3313526400 (2075-01-01 00:00:00 UTC in seconds since epoch)
  - 16000 (2013-10-22 in days since epoch)
  - 18000 (2019-04-14 in days since epoch)

(Sponsored by av.tu-berlin.de and fokus.fraunhofer.de)

Default path for home folder in user_files.sls if home omitted in pillar.

Fix for:
Rendering SLS 'base:users.user_files' failed: Jinja variable 'dict object' has no attribute 'home'