Currently there is no enabling of the heat dashboard if its
installed on the horizon image.
This patch add an extra conf var that allows several dashboards
to be added and will try to find and enable them on start
If the panel dirs dont exists, it will do nothing
This patch add the extra heat_dashboard and includes the existing
neutron_taas_dashboard into the new config
Change-Id: Ibcc4da166d907f3cb842bfc45d842a650361a2d8
55f70be381