Middleware creates tuple on every iteration of a loop

[Ian Cordasco]

In this generator [1] (nested in a for-loop) the middleware constantly recreates the same tuple. This would perform better if two changes were made:

1. Use a set instead of a tuple
2. Assign the set once outside of the for-loop saving several object creations.

[1]: https://github.com/openstack/horizon/blob/0f3192f84c872b1060ac6b7bbe1a3b34bbaf1b36/horizon/middleware.py#L179..L180

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/115691

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/115691
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=961ce7363b865103d022a2119cf1e101557226e2
Submitter: Jenkins
Branch: master

commit 961ce7363b865103d022a2119cf1e101557226e2
Author: Ian Cordasco <email address hidden>
Date: Wed Aug 20 10:43:35 2014 -0500

    Use a singly created set while copying cookies

    Previously a tuple was being created while iterating over each of the
    arguments of a cookie for every cookie. This will prevent creating a tuple
    that many times and uses a set to increase the performance of look-up using
    `in`.

    Change-Id: I10ffccae8eb3a9cb41b1045a08b891fed43efd99
    Closes-Bug: 1359245