In python3, novaclient ListExtResource is not hashable.
An error message on UnhashableKeyWarning is emitted repeatedly
and this can pollute error log with unuseful messages.
A tuple of extension names are now passed to the memoized
decorator so that the decorator can hash it.
Change-Id: I7f200ee6ece36767586207dc26aed1325b909226
Closes-Bug: #1790930