Correct errant uses of label.image.is_diskimage

* nodepool/nodepool.py: In a couple of places
label.image.is_diskimage was incorrectly substituted where
label.is_diskimage was intended. is_diskimage is a boolean attribute
of the Label class, while image is a separate string attribute of
the same class.

Change-Id: Ia142eaeebc5e46086cc2ab0fa290b8d94437dbae