Add deployment on specific instances support for opsworks

Related to #191 

Example :
```
deploy:
  provider: opsworks
  access_key_id: <ACCESS KEY>
  secret_access_key:
    secure: <SECURE SECRET KEY>
  app-id: <APP ID>
  on:
    repo: <REPO>
    branch: master
    instance-ids: 
    - <INSTANCE ID>
```
If you set `option(:instance_ids)`, errors are thrown 
```
Failure/Error: provider.push_app
     DPL::Error:
       missing instance_ids
```

If you set `options[:instance_ids]`, only other tests are failing because the sdk return `:instance_ids=>nil` and they don't expect that.

Do you prefer the second option with updated tests instead of the original PR ?