Improve checking or return values for functions in linuxbridge agent plugin

[Tomasz Paszkowski]

During review process for:

https://review.openstack.org/#/c/26516

we have found that we can improve checking of return values in the following functions:

- ensure_local_bridge
- ensure_vlan

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/27847

[Sandhya Dasu (sadasu)]

ensure_bridge() returns False or None but most functions calling ensure_bridge() donot check if it returned False. Adding these checks as part of this change although not mentioned in the original bug description.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/27847
Committed: http://github.com/openstack/quantum/commit/fa69228606ff42233a3f241a455157e675d398df
Submitter: Jenkins
Branch: master

commit fa69228606ff42233a3f241a455157e675d398df
Author: sadasu <email address hidden>
Date: Thu Apr 25 16:05:22 2013 -0400

    Improve checking of return values for functions in linuxbridge agent plugin

    Fixes bug 1167780

    Changed return values for some ensure_* functions to make them more consistent.
    Now all ensure_* functions return None on failure and return the entity that was
    created within the function when successful. Made changes to the unit tests to
    reflect the changed return values.

    Change-Id: Ib015ee7cee50bae5d91a4e109e7381519c1e14f7