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

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