Refactor allowed provider and clusters sizes configuration

[Jason Mimick]

The broker currently supports an environment variable PROVIDERS_WHITELIST_FILE which is the path to a file containing the cloud providers and cluster instance sizes you wish to make available from your broker endpoint.

For example

 {  
    "AWS": ["M10","M20","M100","M140"],
    "AZURE": ["M10","M20","M200"]
 }

In order to support Cloud Foundry environments the broker should also be able to accept this json directly from an environment variable. We also want to refactor the name of this feature to something more appropriate.

Tasks:

1. Add new env variable to the broker ALLOWED_PROVIDERS which contains either path to file or json
2. Deprecate the existing env, but make it work the same for backward compatibility.

[Jason Mimick]

Closing this issue as not relevant any longs due to design changes with new Plan templates.