discoverd incorrectly reports False from validate when disabled

This currently only affects the "fake" driver, since no other drivers instantiate the DiscoverdInspect class yet. Changing to Medium as it doesn't affect any users.

As I already said in a review, I don't agree it's a bug. We don't have a good way to report unsupported interfaces, None means missing interface, not unsupported. And the crucial difference is whether an admin can "fix" it. Maybe we should invent a "disabled" state?

Reviewed: https://review.openstack.org/161132
Committed: https://git.openstack.org/cgit/openstack/ironic/commit/?id=dc05d18b566174afd772b18e256089a664e4c5b6
Submitter: Jenkins
Branch: master

commit dc05d18b566174afd772b18e256089a664e4c5b6
Author: Dmitry Tantsur <email address hidden>
Date: Thu Mar 5 17:45:43 2015 +0100

    Start using in-band inspection

    Start using in-band inspection with ironic-discoverd, but do not
    instantiate the driver interface when discoverd support is disabled in
    the config file.

    * Check the CONF option prior to instantiating DiscoverdInspect class
      and binding it to any driver interface
    * Updates unit tests to better test the loading (or lack thereof) of the
      driver.inspect interface, based on the CONF option

    If in-band discovery is enabled by setting [discoverd] enabled=True in
    the config file, then the following drivers will bind it to their
    inspect interface:
    * pxe_ssh
    * pxe_ipmitool
    * pxe_ipminative
    * pxe_drac

    This patch also fixes a few nits (copied from previous reviews).

    The bug tagged below presently affects only the "fake" driver. Without
    the above-mentioned changes, enabling discoverd for any
    production-facing driver would have led to the problem described there.

    Updated UnsupportedDriverExtension error message to suggest that
    the extension might be disabled.

    Co-authored-by: Devananda van der Veen <email address hidden>

    Closes-bug: #1431999
    Implements: blueprint inband-properties-discovery
    Change-Id: I1dd844525852b1ec806f286d01dfc95313c6ad94