[fixes #23] '/' vhost is not removed any more.

We still remove the guest user but the new version of
rabbitmq requires the '/' vhost to exist to be able
to restart. There are no security implications with
this change since there is no user to connect to
the default vhost.
Merge pull request #24 from caktus/feature/23-rabbitmq-fails-to-restart

[fixes #23] '/' vhost is not removed any more.