Make foreman integration optional

Instead of including foreman::params, the values are copied. That means 
foreman::params no longer needs to be included.

The next use of foreman is in puppet::server::config where it sets up 
reporting and ENC. This is wrapped in a new boolean $server_foreman. The user
will still need to take care of clearing $server_reports and
$server_external_nodes.

Fixes #154
I'd like to remove it - though so as not to break API immediately, perhaps leave it in the class definition, defaulting to undef and use pick()?  Or we bump the major version, seems excessive right now though.