When rendering a task consul template, ensure that only task environment
variables are used.

Currently, `consul-template` always falls back to host process
environment variables when key isn't a task env var[1].  Thus, we add
an empty entry for each host process env-var not found in task env-vars.

[1] https://github.com/hashicorp/consul-template/blob/bfa5d0e133688920afd1e012404f765182e3d5e0/template/funcs.go#L61-L75