Verify that parsed environment file maps to a dict

An internal unexpected exception is raised while create a stack with an
environment file which only has string content. We should handle the case
and raise the correct error message such as "The environment is not YAML
mapping".

Change-Id: Ie226d122593dd97c3607c269899c0743a9d917d3
Closes-Bug: #1273538
Merge "Verify that parsed environment file maps to a dict"