Speed up scheduler a little

[Matthew Sherborne]

This is a strategy to speed up the scheduler:

These scheduler filters will have predictable results for certain instance_types:

    aggregate_instance_extra_specs, compute_capabalities_filter, core_filter,
    trusted_filter

I propose that the filter be able to return a tuple like:

 ('instance_type', True)

meaning that for this instance type, I'm always going to return True.

Then the filter_scheduler can cache these results, and before it runs through the list of filters, it can check the instance_type and decrease the list of filters to actually call.

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Secondly, these scheduler filters will have predictable results if a certain CONF setting is set:

    isolated_hosts_filter, trusted_filter if it had no 'attestation_server'.

So, basically if they found that they were badly configured, they could log it and return:

('always', True)

To mean that they are always (until the program is restarted) going to allow all hosts.

This assumes that the config will not change while the process is running.

The scheduler can parse these tuples, and remember not to bother calling these filters in future runs.