add self- enable/disable check on plugin load

[Doug Hellmann]

The original architecture design document proposed asking plugins at startup whether or not they should be enabled to avoid polling them later when we know they cannot (or will not) return any useful data. That was never implemented, and needs to be.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/13184

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/14972

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/14972
Committed: http://github.com/stackforge/ceilometer/commit/ae64a792f8ac9a1e0969b205018d219fbaea05db
Submitter: Jenkins
Branch: master

commit ae64a792f8ac9a1e0969b205018d219fbaea05db
Author: Doug Hellmann <email address hidden>
Date: Mon Oct 29 09:31:52 2012 -0400

    Use stevedore to load all plugins

    Move away from calling pkg_resources directly to use
    stevedore to load plugins. This lays the ground work
    for handling disabled plugins during the loading
    phase.

    addresses bug #1021350

    Change-Id: I94c2d0da2a6f9222611716b96455104a71564f29
    Signed-off-by: Doug Hellmann <email address hidden>

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/14973
Committed: http://github.com/stackforge/ceilometer/commit/05d6992ad778e802ef3aa1e406dce7d18606c3a1
Submitter: Jenkins
Branch: master

commit 05d6992ad778e802ef3aa1e406dce7d18606c3a1
Author: Doug Hellmann <email address hidden>
Date: Mon Oct 29 11:41:53 2012 -0400

    Provide a way to disable some plugins

    Provide configuration options for the three plugin sets
    users may want to manage (compute pollsters, central
    pollsters, and notification listeners).

    Extend the plugin API so the extension manager can ask
    each plugin if it should be enabled. This allows, for
    example, the libvirt pollster to be loaded but then
    recognize that it should not be used and disable itself.

    Addresses bug #1021350

    Change-Id: I82da823845ec49c1a93272411c43073bd4954377
    Signed-off-by: Doug Hellmann <email address hidden>