docs: add VPP Container Testbench example and lab

docs: add VPP Container Testbench example and lab

Adding a "VPP container testbench" (pair of Docker containers plus
helper scripts to test Linux and VPP interfaces). Will be part of a
larger set of labs/exercises/tutorials. Putting this baseline setup up
for review first to see if the community sees use/value in it. If so,
additional exercises using the testbench will be added gradually.

Type: improvement
Signed-off-by: Matthew Giassa <mgiassa@cisco.com>
Change-Id: I582310f7355419e907d575f640482ca49cbb282f