Support Variables In Manifest Beyond Input

From what I can tell, use of variables is not supported outside of input fields. Similar to what is being asked for in #251, we need to support variables in other places. More specifically, the `location` field of `dependencies` needs to be able to support a variable to allow passing in user credentials when connecting to a private repository.

One possible alternative to this could be to allow the `deployment.yaml` file to merge in on top of any field in the `manifest.yaml` file (so, in this case, it would replace the `location` field value in the `dependencies` section). That way the `deployment.yaml` file could be encrypted or be generated from encrypted data and used to replace values as needed.