Fixes environment file using correct YAML format

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 "environment file is not Yaml
format".

Change-Id: I4e124db6a8dca9cf811f151ae95058cdaae16a01
Closes-Bug: #1273974
Merge "Fixes environment file using correct YAML format"