Add caching for ec2 mapping ids.

Requests to map ec2 ids can be slow when there are a large number
of instances. These shouldn't ever change so cache them in memory
for a week. The one week timeout is simply to prevent memory from
growing uncontrollably.

Fixes bug 1157437

Change-Id: I3f1c68959a933a7aaa50f122c34a716264c8722d
Make nova.virt.fake.FakeDriver useable in integration testing

With this change nova.virt.fake.FakeDriver can be used for scale testing
using devstack. By setting 'compute_driver = nova.virt.fake.FakeDriver'
in nova.conf and disabiling quota and other limits, we can load up nova
to think it is running 1000's of instances.  This particular scenario is
analagous to how the baremetal driver will work (one machine managing
100's to 1000's of instances).

Make nova.virt.fake.FakeDriver use CONF.host for hostname.

Change-Id: I731ca2968176b4e39dd2fe7fc99f479cc568398b
Merge "Add caching for ec2 mapping ids."
Merge "Make nova.virt.fake.FakeDriver useable in integration testing"