ML2 type and mech managers should use instance vars for drivers lists

Fix proposed to branch: master
Review: https://review.openstack.org/44921

Reviewed: https://review.openstack.org/44921
Committed: http://github.com/openstack/neutron/commit/cd17893dbce60028f0d946f9d30c3c2afea0f2e3
Submitter: Jenkins
Branch: master

commit cd17893dbce60028f0d946f9d30c3c2afea0f2e3
Author: Dane LeBlanc <email address hidden>
Date: Tue Sep 3 13:34:42 2013 -0400

    ML2 type and mech managers should use instance vars for drivers lists

    Fixes bug 1220296

    The ML2 plugin's type and mechanism managers currently maintain
    dictionaries/lists of type drivers, mechanism drivers, and ordered mechanism
    drivers in (static) class variables. Once a type/mechanism/ordered-mechanism
    driver of any given type is added to this list, then no new drivers of that
    type are allowed to be registered, and therefore no new configuration for
    that driver type is accepted.

    This static nature of the driver dictionaries/lists is causing ML2 mechanism
    driver unit test cases to fail. For example, if a non-vendor-specific ML2
    plugin test case configures a VLAN type driver with no VLAN range, and then
    a vendor specific test case attempts to configure a VLAN type driver with
    some test VLAN range, then the new VLAN configuration is ignored because of
    the previously (staticly) registered VLAN driver.

    The proposed fix is to convert these driver dictionaries/lists to instance
    variables, and clear them upon each instantiation of an ML2 type manager
    or ML2 mechanism manager.

    Change-Id: I3b5209640de229899561e2a3ec7c6dafe9a05e64