federated login - enable/disable each provider individually in config

[peta15]

add ability to choose which federated providers you want to support via the config. I had originally intended to add this feature but it was put on the backburner so right now we only have a global enable_federated_login flag. We'll replace this with an array of which providers to enable much like the languages array in the config.

Thanks Emmanuel for reminding me of the need for this feature!

[peta15]

also add to the array 'local' which will enable/disable local login and have a check to ensure the array is not empty. Or create a separate enable/disable local login setting.

[coto]

This is a very good idea. I would like to change the config file to ConfigParser that is more standard in Python projects and with this we can add an array of federated login enabled.