Bump FakeDriver's resource numbers

Bump the amount of memory and disk that FakeDriver reports so one can
schedule larger instances. One should be able to schedule an instance
larger with more then 8GB of ram onto the FakeDriver.

Since the FakeVirt driver is now used for much more then just unit
tests, subclass FakeVirt to decouple the amount of resources FakeVirt reports
from the amounts in the api_samples. This way the api_samples don't need changing every
time the resource amounts reported by FakeVirt are tweaked.

Change-Id: Ifd3b8bd1abf985d279de334776ed5feb4fa72a27
Related-Bug: #1268943
Merge "Bump FakeDriver's resource numbers"