* 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'

This version complains that "argument port can not be used in
conjunction with argument hash_hostname", so add hash_hostname
to the fields we handle in the formula so we can override it
if needed.

sry, could not resist.

This formula doesn't really require the sudo group (unless
there are actually users in that group). Moreover, on FreeBSD
the 'admin' group would be wheel and not sudo.

This reverts commit a0392693e3.

Fixes #82.

This will allow users change their passwords after the initial setting in Salt.

SSH key pairs deployed via the user's ssh_keys_pillar dict aren't
handled the same as the user's ssh_keys, e.g., file ownership and
permissions aren't specified, and the keying material gets copied
directly into the SLS file.  This change rewrites the two templated
file.managed states to behave as follows:

  - set the files' owner to be the user

  - set the files' group to be the user's primary group

  - for the public key, set the mode to 644 (u=rw,go=r)

  - for the private key, set the mode to 600 (u=rw,g=)

  - pull the files' contents directly from pillar

Signed-off-by: René Jochum <rene@jochums.at>