Use heat-manage to initialize Heat db.

Since this commit https://review.openstack.org/#/c/30580/
calling the heat.db.sync module no longer works
and firstboot fails with "No module named heat.db.sync".

Also depends on this patch in heat to fix heat-manage
https://review.openstack.org/#/c/30690/

Change-Id: Iaf862e03a1f3f0ed2d2ddd83a8ead2a5f287789e