Remove default username for sensu

In some cases we need a passwordless sensu dashboard for different reasons, for example if we would like to manage the Auth using Nginx + Google_Auth_proxy[1].

Adding the default sensu user on the provider would mean that we have to authenticate everytime which doesn't make much sense.

[1] https://github.com/bitly/google_auth_proxy
Merge pull request #214 from rhoml/rhoml/add-passwordless-dashboard

Remove default username for sensu