* don't load every possible answer, only do what is needed
* cache instance data for a given address for a 15 seconds
using either memcache or fake memcache (in-memory).
This means only a single queue/db lookup for multiple calls
to metadata service
* add cache expirey to fake memcache (don't grow forever)
and move it to nova.common.memorycache
Addresses Bug #851159
Change-Id: Icf794156e055b18915b8b5be9ba2ab97d2338bbe
610df5e87d