Fix static strings in DVR config with LABELs

[Swaminathan Vasudevan]

There are couple of static strings in DVR that are used for checking the configuration modes of the L3-agent such as "dvr_snat", "dvr", "legacy", "agent_mode" and "distributed".
We need to replace those with LABELs.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/147605

[OpenStack Infra (hudson-openstack)]

Change abandoned by Swaminathan Vasudevan (<email address hidden>) on branch: master
Review: https://review.openstack.org/147605
Reason: Redundant patch.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/143568
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=171d4a6f0066ab29bc5533b0bf85c5daa9f17507
Submitter: Jenkins
Branch: master

commit 171d4a6f0066ab29bc5533b0bf85c5daa9f17507
Author: Swaminathan Vasudevan <email address hidden>
Date: Fri Dec 19 16:58:17 2014 -0800

    Fix static strings with labels for DVR

    There are couple of static strings introduced for
    DVR configuration such as the L3 agent mode.

    This patch would clean up the static strings with
    the labels.

    Closes-Bug: #1411351
    Change-Id: I143262e62ee994e6c01d63e4ea7d5f6292de8286