Update ML2 README file for havana

Closes-Bug: 1235308
Change-Id: I684581c00ff28bc9cdb36034a9a970275b0a0090
(cherry picked from commit 2d7e2756271fff6413a572e11f9248465f23f1bb)
Add clear_db to cleanup for TestCiscoNexusPlugin

Fixes Bug: #1234012
Change-Id: I0829f04ac1a0760d96babcb1154f4f569a0f848f
(cherry picked from commit b4167ed095dbd47a137ff22674ae13aba3a50513)
Merge "Update ML2 README file for havana" into milestone-proposed
Merge "Add clear_db to cleanup for TestCiscoNexusPlugin" into milestone-proposed