Replace hardcoded strings with consts in fuelmenu

[Matthew Mosesohn]

We use hardcoded strings in fuelmenu for things like "pre" and "post" deployment modes. This should be located in a consts class and referenced symbolically.

[Matthew Mosesohn (raytrac3r)]

From Andrey Tykhonov in https://review.openstack.org/#/c/242109: Could you please get rid of these magic (and surplus) constants? Could you please create two methods instead, something like `is_pre_deployment`, `is_post_deployment` (and add doc-strings for them)?

[Matthew Mosesohn (raytrac3r)]

TODO in this bug: move versionfile and astute_yaml paths to constant strings as well

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/259983

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/259983
Committed: https://git.openstack.org/cgit/openstack/fuel-menu/commit/?id=98f108c2c7eeaff5181e0b39b356b4b99bf86f86
Submitter: Jenkins
Branch: master

commit 98f108c2c7eeaff5181e0b39b356b4b99bf86f86
Author: Matthew Mosesohn <email address hidden>
Date: Mon Dec 21 15:45:26 2015 +0300

    Add consts to fuelmenu

    logfiles, config files, version files, and deployment
    modes are now stored as constant values.

    Added is_(pre|post)_deployment methods to simplify
    logic when looking up deployment mode.

    Change-Id: I114e0a82fa07ff3bdcf0d0c4fb37d94d05acd923
    Closes-Bug: #1514382