Need to install python-heat-dashboard in Queens

[Corey Bryant]

Heat was removed from horizon in Queens:
https://github.com/openstack/horizon/commit/eac3e7032a48fb9f03f56af0cc2e2085d55af0d0

As a result the charm will need to install the new package python-heat-dashboard.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/543452

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/543452
Committed: https://git.openstack.org/cgit/openstack/charm-openstack-dashboard/commit/?id=cd8b510f95e2e7d17ba5c3639ff7239d1ba92b4b
Submitter: Zuul
Branch: master

commit cd8b510f95e2e7d17ba5c3639ff7239d1ba92b4b
Author: Corey Bryant <email address hidden>
Date: Tue Feb 6 15:17:43 2018 +0000

    Enable Heat and Designate dashboards in Horizon

    The Heat dashboard was split from horizon in Queens, and
    therefore needs to be installed separately.

    The Designate dashboard was never installed by the charm
    before but should be.

    Change-Id: Ie0790fd1e1a6422465f4a4e00769efcd667d28e2
    Closes-Bug: #1746031
    Closes-Bug: #1747934