Adding aggregates panel to admin dashboard

[Deepti Navale]

With the Icehouse release, admin users can add and modify 'Host Aggregates' using the dashboard. Include the procedure to do the same in the guide.

For more details:
Blueprint - https://blueprints.launchpad.net/horizon/+spec/manage-host-aggregates

Dev bugs and the merged patches:
https://bugs.launchpad.net/horizon/+bug/1197775 - https://review.openstack.org/35663
https://bugs.launchpad.net/horizon/+bug/1261932 - https://review.openstack.org/71061

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/86489

[Openstack Gerrit (openstack-gerrit)]

Reviewed: https://review.openstack.org/86489
Committed: https://git.openstack.org/cgit/openstack/openstack-manuals/commit/?id=9da6bf5b4d45f5d3a7b65c84343247e343c4c49b
Submitter: Jenkins
Branch: master

commit 9da6bf5b4d45f5d3a7b65c84343247e343c4c49b
Author: Deepti Navale <email address hidden>
Date: Thu Apr 10 12:11:39 2014 +1000

    Included new section 'Create and manage host aggregates'

    New section to the Admin User guide to 'Create and manage host aggregates'
    in the dashboard chapter.
    Partial-Bug: #1304821

    Change-Id: If2aa8cedaf7a3510123328b11406c9cb60e3fccf