Made the auth backend customizable with attributes

[djolly-fuze]

Changed some auth config settings to attributes to allow for for customized auth options.

[arm4b]

@djolly-fuze One missing thing I noticed a bit late, can you also update other files to reflect this change and make unit tests pass?

See

• https://github.com/StackStorm/chef-stackstorm/search?q=auth_standalone_file&unscoped_q=auth_standalone_file
• https://travis-ci.org/StackStorm/chef-stackstorm/jobs/