Expand check on 'environment' argument for BaseClient class

[appukuttan-shailesh]

The environment argument for the BaseClient class currently accepts only 'production' and 'dev' as valid values. This should be extended to allow users to specify their own values. Parameters appropriate for these would then be read from an external config file.

[appukuttan-shailesh]

@apdavison : is the following structure fine for the config file?

{
  "dev1": {
      "url": "www.....",
      "client_id": ""
  },
  "dev2": {
      "url": "www.....",
      "client_id": ""
  }
  ...
}

Do you foresee the need for any other parameters here at this point? username, password?

[apdavison]

Looks fine to me. We can always add further parameters later.

[appukuttan-shailesh]

This is the method header currently: def __init__(self, username, password=None, environment="production"):

We plan to access the config file for any values of environment apart from "environment" and "dev". Should we force a name and location for the config file (such as environ.json in the current directory), or add some new parameter for specifying the config path?

[apdavison]

I suggest config.json in the current directory.