Updated documentation for openvpn::client subclasses

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>
Fixed pidfile path on Ubuntu 14.04

Ubuntu 14.04 openvpn packages keep the pidfile in /var/run/openvpn instead of
/var/run.

This change also makes params.pp fail if an unsupported OS is detected, and also
makes the openvpn class inherit openvpn::params instead of including it. This
means including the class on an unsupported OS will fail immediately _before_
puppet attempts to make any configuration changes.

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