Handle username for strongops command

cc @sam-github 

Take username for authentication or new registrations.

Adjusted the error handling based on NodeFly/nodefly-registration now
always returning an error object for errors.

Changes messaging to communicate that the username is the login for
either strongloop.com or strongops.

Can be tested against new api on staging by setting `SL_OPS_REGISTER_API=https://strongops-staging.strongloop.com/ops/rest/`