Fixes to cope with ansible 1.4 changes

It seems that ansible_ssh_user and 'sudo' are not friends anymore on
ansible 1.4 (ref: ansible issue #5030)
Fixes #26
Adds explanation about difference README and hosts

hosts file described in the README is not exactly the same. This is
because, if we don't specify a user, ansible will use it's defaults
(remote_user in ~/.ansible.cfg) and the playbook potentially won't work
for some people.
This forces ssh'ing as root, and fixes this potential problem.
Fixes #25
Improves description of ansible_ssh_user

Added a bit more information on why ansible_ssh_user is used.