Expounded on global environments.
Expounded on global environments.

Explained that the effective environment used for a stack is the
combination of the user-supplied environment and the provider-supplied
"global" environment.

Change-Id: I0e005dad6efb80a76ecff6337454e2e891e692ca
Partial-Bug: #1297060

rh-s/heat,maestro-hybrid-cloud/heat,pshchelo/heat,noironetworks/heat,steveb/heat,miguelgrinberg/heat,redhat-openstack/heat,gonzolino/heat,cryptickp/heat,jasondunsmore/heat,pshchelo/heat,cwolferh/heat-scratch,rdo-management/heat,dims/heat,cryptickp/heat,openstack/heat,steveb/heat,takeshineshiro/heat,dims/heat,jasondunsmore/heat,dragorosson/heat,maestro-hybrid-cloud/heat,dragorosson/heat,srznew/heat,openstack/heat,miguelgrinberg/heat,gonzolino/heat,noironetworks/heat,redhat-openstack/heat,rdo-management/heat,rh-s/heat,takeshineshiro/heat,pratikmallya/heat,srznew/heat,pratikmallya/heat,cwolferh/heat-scratch