stevedore no longer raises KeyError for missing plugin

[aeva black]

Stevedore issue #15 ( https://github.com/dreamhost/stevedore/issues/15 ) was recently closed, meaning that NodeManager._*_factory.map() will stop raising KeyErrors for missing drivers. We should be able to stop ignoring KeyErrors now:
  https://github.com/openstack/ironic/blob/master/ironic/manager/resource_manager.py#L69

There should probably also be a unit test to confirm the load continues to work without needing a try: block, even with missing plugins.

[aeva black (tenbrae)]

Confirmed that as of stevedore 0.13, extension_manager continues to raise KeyError for plugins which are not found.

The linked github issue seems to be gone (or it moved and I don't know how to find it). Wrapping the stevedore call inside our driver_loader to catch KeyError has been working fine thus far, so I am going to close this bug and continue to use the existing approach.

[Michael Davies (mrda)]

I played with this one as well - can't reproduce the described stevedore behaviour, so marking this as invalid seems like a good idea.