Make plugin_parent_dir creation a configurable option

The management of the basic dependencies of a Jenkins plugin should not
be unnecessarily forced on the user.

This change adds a new boolean parameter, `$manage_dependencies`, to
allow the user to specify whether or not jenkins::plugin will create
these dependencies. The default is true to support backwards
compatibility.

When set to false, the `/var/lib/jenkins` directory, the jenkins user
and the jenkins group are expected to be defined elsewhere or already
exist.

In addition, the plugin_parent_dir resource is checked for a previously
defined resource before being created in the same way that the
plugin_dir, user and group are checked.

Specific tests were created for the new parameter and they verify that
plugin_dir is created whether `$manage_dependencies` is true or false.

This fixes #148 in a more backwards compatible way than #149.