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
 Arista ML2 Mechanism driver

This patch implements Arista's modular L2 mechanism driver to
automate the management of virtual networks along with physical networks
using Arista hardware devices (Spine and Leaf switches)

This driver uses ML2 Mechanism Driver-API to interface with Neutron ML2 Plugin.

implements: blueprint arista-ml2-mechanism-driver

Change-Id: I1c4ca36cf3d7af013b7b3353e7b7d89fe39f91bf
Merge "ML2 type and mech managers should use instance vars for drivers lists"