Add defined check for plugin_parent_dir resource

The management of the basic dependencies of a Jenkins plugin should not
be unnecessarily forced on the user. This change wraps the plugin_parent_dir
resource creation in a `defined` function to allow the user to create the
Jenkins home directory themselves if necessary.

The use case for this would be a simple symbolic link resource that would link 
the default `/var/lib/jenkins` parent directory to another location for storage 
space, or other system reasons.

This fixes #148 in a more backwards compatible way than #149.