Arista ML2 mech driver cleanup and integration with portbindings

Arista ML2 mech driver cleanup and integration with portbindings

fixes bug: 1215097

This patch addresses minor cleanups which are the diff between the
original version of the patch which was merged into havana master
branch and the patch that is submitted for merge into stable/havana.

The original patch that was merged into havana master branch is at:
https://review.openstack.org/#/c/49086/
The new patch which has been submitted as havana-backport-potential is:
https://review.openstack.org/#/c/54447/

This whole excercise came into play because the original patch was not
picked up in havana-rc2 because of missing tag in the bug. Therefore, the
need to submit new patch against stable/havana.

When the new patch was submitted for havana-backport-potential, few very
minor previously commented cleanups were incorporated as well. This
patch brings these minor cleanups into master branch as well. Therefore,
merge of these two patches will bring both master branch and
stable/havana at parity.

Change-Id: I4ec5273c26a27ca52b430a6c7485a44e880a1787