Update the ML2 configuration in devstack

This commit adds support to devstack to set ML2 options including
MechanismDrivers, TypeDrivers, and TypeDriver specific configuration for ML2
when running with devstack.

This also allows for simple configuration of VLAN, GRE, and VXLAN ML2 devstack
setups with the OVS agent as follows:

    # VLAN configuration
    Q_PLUGIN=ml2
    ENABLE_TENANT_VLANS=True

    # GRE tunnel configuration
    Q_PLUGIN=ml2
    ENABLE_TENANT_TUNNELS=True

    # VXLAN tunnel configuration
    Q_PLUGIN=ml2
    Q_ML2_TENANT_NETWORK_TYPE=vxlan

Fixes bug 1200767

Change-Id: Ib16efac13440b5d50658a0e6be35bc735510a262
Merge "Update the ML2 configuration in devstack"