This allows using the environment values defined in the environments: section of helmfile.yaml to be used from other sections of the file.
This works by having two template renderers, the first-pass and the second-pass renderer.
The first-pass render renders a helmfile.yaml template with replacing template functions has side-effects with noop. So, use only funcs that don't have side-effects to compose your environment values.
Then the second-pass renderer renders the same helmfile.yaml template, but with the environment values loaded by the first-pass renderer.
The implementation uses a buffer instead of re-reading the file twice.
Resolves#297
David Genest
KUOKA Yusuke