Add support for env. variables as a YAML key

We just verified that we do have support for env. variables in #249 and they can be referenced by using ($) dollar notation like `$CLOUDANT_USERNAME` and `$CLOUDANT_PASSWORD` in the following example:

```
package:
name: openwhisk-data-processing-cloudant
namespace: _
dependencies:
# binding cloudant package named $CLOUDANT_INSTANCE
$CLOUDANT_INSTANCE:
location: /whisk.system/cloudant
inputs:
username: $CLOUDANT_USERNAME
password: $CLOUDANT_PASSWORD
```

But we do not support env. variables when they are referenced by the YAML keys for example, `$CLOUDANT_INSTANCE` in the above sample. We do have such use case as part of [openwhisk-data-processing-cloudant ](https://github.com/IBM/openwhisk-data-processing-cloudant) repo.

@lionelvillard @mrutkows @paulcastro @krook