Aer backend giving depreciation warnings using terra master

[nonhermitian]

Informations

• Qiskit Aer version: master
• Python version:
• Operating system:

What is the current behavior?

/miniconda3/lib/python3.7/site-packages/qiskit/providers/aer/backends/aerbackend.py:119: DeprecationWarning: The as_dict() method is deprecated, use to_dict().
  config = original_config.as_dict()
/miniconda3/lib/python3.7/site-packages/qiskit/providers/aer/backends/aerbackend.py:54: DeprecationWarning: The as_dict() method is deprecated, use to_dict().
  return obj.as_dict()

Steps to reproduce the problem

What is the expected behavior?

Suggested solutions

[atilag]

We do have a NoiseModel::as_dict() method that we may want to change to .to_dict() the same way Terra did, otherwise our AerJSONEncoder class won't know how to serialize NoiseModel objects.

This is changing the API of an important component, just making sure @chriseclectic and @gadial are fine with this change, and what implications have (more than changing the tutorials).
I'll put a depreaction warning in our NoiseModel::as_dict() method so we can remove it for 0.3.x or 0.4.x.

[atilag]

For the sake of consistency, I have changed all our .as_dict() API methods, to to_dict().