Use axios supplied basic auth functionality

[nwinch]

Currently basic auth is implemented with custom headers as an interceptor for all requests, though axios can support basic auth as an options - from the docs:

// `auth` indicates that HTTP Basic auth should be used, and supplies credentials.
// This will set an `Authorization` header, overwriting any existing
// `Authorization` custom headers you have set using `headers`.
auth: {
  username: 'janedoe',
  password: 's00pers3cret'
}

I suggest we change the auth-driver opts to match axios opts. So this would change:

{
  password: 'secret'
}

to...

{
  auth: {
    username: 'user',
    password: 'secret'
  }
}

By doing this it would remove unnecessary code as the functionality is supported natively.