(#7656) Add new PMT settings to core Puppet

Before this patch, Puppet core did not support any settings for the
Puppet Module Tool (PMT).

This patch adds two (2) new settings to core Puppet:

• module_repository
• module_working_dir

The module_repository setting is used to specify the remote module
repository used for searching and installing modules; module_repository
has a default value of 'http://forge.puppetlabs.com'.

The module_working_dir setting is used to specify the directory into
which module tool data is stored; module_working_dir has a default value
of: '$vardir/puppet-module'.