add help tooltip to column headings

[Cindy Lu]

Based on Liz's suggestion (https://bugs.launchpad.net/horizon/+bug/1267362):

"If a clear, concise term can not be created I think the idea of adding a help "?" icon with hover over text next to the column header would be a fine solution. I don't know that we do this anywhere yet, but it would be something that could be added and used throughout Horizon where needed. I think simply adding the "?" icon would not clutter things too much."

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Add help tooltip if user specifies it when creating a Column.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/128804

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/128804
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=ca1048c193136493850409fa4c8ed5ce5a193661
Submitter: Jenkins
Branch: master

commit ca1048c193136493850409fa4c8ed5ce5a193661
Author: Cindy Lu <email address hidden>
Date: Wed Oct 15 18:12:30 2014 -0700

    add Column attribute for help tooltip in heading

    To use: add attribute help_text=_("foo") to tables.Column(...)
    This will add a help icon next to the Column name which
    you can hover-over to get informational text.

    Change-Id: I10a28bd51efbaf83c617b30b31d861445418f9d8
    Closes-Bug: #1381633