produce warnings for env. variable outside of inputs section?

With adding new concept of inputs section in manifest/deployment file, whisk deploy produces failure in case of an input is required but its value (env. variable) is not set. With this new functionality, there is no need for producing this kind of warning:

```
wskprint.PrintlnOpenWhiskWarning("Missing Environment Variable " + substr + ".")
```

I have disabled this warning, the only catch is when a variable is referenced `$USERNAME` in manifest/deployment but not defined under `inputs` section and not set in env. will go without noticing or producing any warning. Should we assume that this kind of usage is no longer supported?

Start with the whisk deploy handling for manifest/deployment files and the environment-variable warning shown in the issue. Determine whether variables outside the inputs section should be supported or warned about, then document the chosen behavior and verify the missing-input and environment-variable cases.

Written by the indexing model from the issue text.