Add the ability to completely delete a floating IP from the pool
when disassociating it from a server. When state is absent and
purge is true, the IP will be completely deleted. The default
keeps the current behavior, which is to only disassociate the IP
from the server.
The exception message, when shade fails, will contain much more
specific information about the failure if the exception is treated
as a string. The 'message' attribute alone is usually not helpful.
