Added ability to use hiera_hash to populate users

If one were to set ::stackable_base::users to 'hiera_hash'
puppet will use the hiera_hash() function to merge a hash
of users provided in the ::stackable_base::user_hash key
from hiera.

Added fake spec test for this, as I cannot get
hiera-helper-puppet or hiera-rspec-puppet working to verify
the installation.
Added ability to specify sshd_config in hiera

What it says on the tin. If stackable_base::sshd_config is set
in hiera, puppet will use it.

If not, puppet will fallback on safe-defaults
Corrected typo in documentation.
Added nscd class and spec checks.

Added nscd.pp, installs and starts nscd service.
Added stackable_base::services namespace

Created stackable_base::services namespace to make including
single-purpose service configs easy. Moved ssh and nscd to this
new namespace.