Removed the $ssl_dir variable from class parameters

Different operating systems and installation methods put the Puppet certificates
to different places by default. Usually we don't have any reason to diverge from
those defaults, and we'd prefer to keep OS knowledge in params.pp where it
belongs, instead having (some of) it in the node definitions. Added new variable
$::puppetagent::params::ssl_dir to abstract away the operating system
differences.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>
Updated LICENSE copyright to 2013

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>