db.compute_node_update: ignore values['update_at']

When individual instances are updated (e.g., during spawn and
terminate), ResourceTracker (in nova.compute.resource_tracker)  calls
compute_node_update with values=self.compute_node. Since
self.compute_node is an instance of ComputeNode that was retrieved
from the database, it has updated_at set. Since updated_at is in
values, sqlalchemy doesn't automatically change the record's
updated_at column (see
nova.openstack.common.db.sqlalchemy.models.TimestampMixin). Moreover,
since updated_at is set to the last value's updated_at, updated_at
effectively doesn't change until values without updated_at are sent,
which only happens during the periodic task that calls
ResourceTracker.update_available_resources.

Nova-scheduler relies on ComputeNode.updated_at to keep its model of
available resources up-to-date. In particular, nova-scheduler doesn't
play a role in instance termination, so it doesn't account for freed
resources until ComputeNode.updated_at changes. Thus, between
nova-compute's periodic calls to
ResourceTracker.update_available_resources, nova-scheduler's model of
available resources monotonically decreases. If, for example, a node
has resources for 10 instances, and you manage to boot 10, terminate
10, then attempt to boot another before the end of the period,
nova-scheduler won't schedule the new instance on the vacant node.

Fixes bug #1194900.

Note that f398b9e195cda582bad57396b097dec274384c07 fixed a separate
issue (bug #1153778) related to ComputeNode.update_at being stale.

Change-Id: Ifd1e56edfd811241816970715071876857de80d3

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