Module string has no more attribute letters in python 3

[François Magimel]

According to the Python doc (https://docs.python.org/3.0/whatsnew/3.0.html), string.letters is not anymore in Python 3 due to its locale-specific behavior. Instead, it is recommended to use string.ascii_letters.
This method is used only in the generate_key function (in horizon/utils/secret_key.py) to generate a random key for the settings.py file.

[Akihiro Motoki (amotoki)]

Hopefully it is nice to have Hacking rule to detect Python3 compat. It helps us to do the similar thing manually in the future.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/116516

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/116516
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=84d8e08080ea7f81356a4bee1e90e39b86e133d3
Submitter: Jenkins
Branch: master

commit 84d8e08080ea7f81356a4bee1e90e39b86e133d3
Author: François Magimel <email address hidden>
Date: Sun Aug 24 20:22:55 2014 +0200

    Use string.ascii_letters to be python 3 compatible

    Use string.ascii_letters instead of string.letter. The only consequence
    is to loose the locale-specific behavior but it seems not to be a bad
    thing for generating a secret key.

    Change-Id: Id985d798042eaa7541692d137027233537ed3bb4
    Closes-Bug: #1360861