Add Geneve type driver support to ML2

Add Geneve type driver support to ML2

More information about Geneve protocol can be found here:
  https://tools.ietf.org/pdf/draft-gross-geneve-02.pdf

Following configuration variables were added:
[ml2_type_geneve]
  vni_ranges - Comma-separated list of <vni_min>:<vni_max> tuples
               enumerating ranges of Geneve VNI IDs that are
               available for tenant network allocation
  max_header_size - Geneve encapsulation header size is dynamic, this
                    value is used to calculate the maximum MTU for the driver
                    this is the sum of the sizes of the outer
                    ETH + IP + UDP + GENEVE header sizes

DocImpact

Change-Id: I8c29a1c1a7c79e02c26ac9e2ad2645d30dfbeefc
Closes-Bug: #1461069