Migrate to oslo.context

[Ihar Hrachyshka]

A new oslo library graduated: oslo.context. We should migrate to use its RequestContext.

Documentation: http://docs.openstack.org/developer/oslo.context/api/context.html

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/141144

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/141144
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=089e60a68b313c1f7077f010964ac27141425517
Submitter: Jenkins
Branch: master

commit 089e60a68b313c1f7077f010964ac27141425517
Author: Ihar Hrachyshka <email address hidden>
Date: Thu Dec 11 20:36:48 2014 +0100

    Migrate to oslo.context

    That's just a matter of adding requirements.txt entry, fixing imports,
    and dropping the corresponding oslo-incubator module from the tree.

    While at it, made all imports to import the module into 'oslo_context' and
    not just 'context', so that we don't override the module locally in
    multiple methods that receive their context arguments with the same
    name, making the library inaccessible from inside those methods.

    Change-Id: Ie62af970b3b7f225de453e56c01abc4b12af8f5e
    Closes-Bug: #1401054