Clear public address orphan count on allocation - EUCA-7995

The orphan count for public addresses is now cleared to ensure that any
previous count is not applied to the current allocation. This could have
caused premature clean up of addresses after an (unrelated) unsuccessful
instance launch.

EUCA-7995
Merge remote-tracking branch 'origin/dev/sjones/EUCA-7995' into maint/3.4/testing