Create a new directory for Arista drivers

[Sukhdev Kapur]

When we created neutron/plugins/ml2/drivers/mech_arista, the intent was only to develop ML2 Mechanism driver. Now that we are getting ready to develop additional drivers, it does not make sense to move them into mech_arista directory. Therefore, the plan is to rename mech_arista to arista (a more generic name) - so that additional drivers can be moved into this directory and the code can be shared between the drivers.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/108561

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/108561
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=78bb3d2be8e944f8832534c861da74899b404d29
Submitter: Jenkins
Branch: master

commit 78bb3d2be8e944f8832534c861da74899b404d29
Author: Sukhdev <email address hidden>
Date: Mon Jul 21 19:28:17 2014 -0700

    This patch changes the name of directory from mech_arista to arista

    This name change of directroy is needed so that additional drivers
    can be moved into this directory and share the code among drivers.

    In order to accomodate this name change, setup.cfg is modified to
    point to the Arista ML2 driver at the new location. Additionally,
    a similar new directroy is created for test files as well.

    Change-Id: I614ccf8b6d152aa99e20ef1ea762c26115ab9373
    closes-bug: 1346712