Add networking scenarios for ml2 with l2pop

This patch add a new section for the ML2 plugin
in networking scenarios section.

Also add 2 first scenarios with ML2 and the
L2 population mechanism driver.

Closes-Bug: #1252703
Change-Id: Iac01b03cb94b1bddc75a3f1fa81c198914ee49ea
(cherry picked from commit 84f4aa0f79fb096eacc6b74e740e4562a5dff653)
Merge "Add networking scenarios for ml2 with l2pop" into stable/havana