Add use case to USECASES.md

	It is a use case listed in https://wiki.openstack.org/
	wiki/Puppet-openstack/ceph-blueprint

	Adding the ceph blueprint in USECASES.md

	Adding a link in the README.md

Closes-Bug: #1243333

Change-Id: Iff1415cace170676c9ff8ec57d05d9bd6bf8ccdf
Blueprint: Add the "I want to spawn a cluster configured with continuous integration" use case to USECASES.md
Signed-off-by: Romain Salot <romain.salot@gmail.com>