Heat has an additional configuration for plugin_dirs
parameter. This parameter provides a list of directories
to search for plug-ins. This change allows configuration
of plugin_dirs parameter in heat.conf file. This change
will allow a user to set this value, if required. Else
$::os_service_default will be used and the parameter will
not be added to the config file, as it is done today.
Change-Id: I636d52f867ee447eaf0e1e80bf9fdc30c91f4ec1