docker: Run mistral-db-manage populate at step 5
docker: Run mistral-db-manage populate at step 5

This resolves issues in loading some of Mistral openstack
actions which appears to require endpoints to be running first
before fake clients can be initialized properly.

Change-Id: Ia588e5ec8880da1df95a33696b5b8c9e72ac49e2
Closes-bug: #1728682

dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates