Cisco APIC ML2 mechanism driver, part 2

    This set of changes introduces a mechanism driver for the
    Cisco APIC. This is the second and final part of a 2 part commit.
    Please see the blueprint for more information.

    The review is submitted in two parts:
    - Part 1 (Posted earlier, required for Part 2)
        o APIC REST Client
        o APIC data model and migration script
        o APIC configurations
    - Part 2 (this commit)
        o APIC mechanism driver
        o APIC manager

    Partially implements: blueprint ml2-cisco-apic-mechanism-driver

Change-Id: I5ed3ac133146635083e2d0093057b43b64f347fe