Refresh network cache when reassigning a floating IP in Neutron

When Nova associates or disassociates a floating IP address
the network info cache is always refreshed due to the
@refresh_cache decorator

However if you (re)associate a floating IP which is already
associated with another instance on a Neutron based system then
the address is correcly disassociated first, but the cache refresh
is not trigged for the original  instance. As a result Nova will
show the same address for two instances until the periodic
clean-up kicks in some minutes later.

Fixed bug: 1173227

Change-Id: Iea81532911478026b9b2c0578a1eda7a97b09d80
Merge "Refresh network cache when reassigning a floating IP in Neutron"