Make the login prompt configurable

The different platforms use different kind of logins:
 - cloudcontrolled uses 'Email'
 - dotcloudapp uses 'Email'
 - app.exo.io uses 'Email or Organization ID'
 - cnh-apps uses 'Login'

Especially on cloud&heat the login is a combination of tenant_name
and email. Asking only for the email confuses the customers (and me).
Merge pull request #35 from cloudControl/differentlogin

Make the login prompt configurable