Add a crowbar-openstack cookbook

The goal is to collect helpers that are useful accross all OpenStack
cookbooks, but don't belong to the core of Crowbar.

We start with helpers to fetch the information about the database server
and the rabbitmq server.
Avoid search if current node matches what we look for
Change log level to warn when no node is found

This can be bad sign, so warning is more appropriate.
Merge pull request #75 from vuntz/crowbar-openstack

Add a crowbar-openstack cookbook