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.

This fixes #148 in a more backwards compatible way than #149.
Add slave service notification when defaults file changes
Remove echo debug line from slave service script

This removes what looks like an errant debug echo from the Debian slave
service script.
Add missing slave mode to Debian defaults file

This fixes #154.
Merge branch 'slave_service_notification_fix' into production
Merge branch 'slave_mode_debian_fix' into production
Merge branch 'slave_service_echo_fix' into production