engine : perform periodic tasks with stored stack_context
engine : perform periodic tasks with stored stack_context

The metadata update needs to be performed with the stored stack_context,
not the admin_context, which contains no credentials to e.g authenticate
with nova

Fixes bug #1196618

Change-Id: I0451604535a731de5ad57d2d52cdf183e58cf7b0

steveb/heat,ntt-sic/heat,rh-s/heat,takeshineshiro/heat,maestro-hybrid-cloud/heat,miguelgrinberg/heat,citrix-openstack-build/heat,NeCTAR-RC/heat,cryptickp/heat,noironetworks/heat,pshchelo/heat,varunarya10/heat,rickerc/heat_audit,gonzolino/heat,NeCTAR-RC/heat,openstack/heat,gonzolino/heat,cwolferh/heat-scratch,JioCloud/heat,varunarya10/heat,ntt-sic/heat,rdo-management/heat,rdo-management/heat,pshchelo/heat,maestro-hybrid-cloud/heat,miguelgrinberg/heat,dragorosson/heat,openstack/heat,jasondunsmore/heat,dragorosson/heat,srznew/heat,redhat-openstack/heat,rickerc/heat_audit,citrix-openstack-build/heat,jasondunsmore/heat,JioCloud/heat,pratikmallya/heat,pratikmallya/heat,dims/heat,cwolferh/heat-scratch,srznew/heat,dims/heat,rh-s/heat,noironetworks/heat,takeshineshiro/heat,cryptickp/heat,steveb/heat,redhat-openstack/heat