nightly.conf: fix is_milestone property.
nightly.conf: fix is_milestone property.

s/poky_name/is_milestone/

Signed-off-by: Beth Flanagan <86e3219095765dcc0f6f42aef4bd43803a05b796@intel.com>

denny820909/builder,denny820909/builder,denny820909/builder,denny820909/builder