Nicolas, I have the same setup dedicated physical system running engine on CentOS 6.6 three hosts running CentOS 7.1 with Gluster and KVM, and firewall is disabled on all hosts. I also followed the same documents to build my environment so I assume they are very similar. I have on occasion had the same errors and have also found that "ctdb rebalanceip <floating ip>" is the only way to resolve the problem. I intend to remove ctdb since it is not needed with the configuration we are running. CTDB is only needed for hosted engine on a floating NFS mount, so you should be able change the gluster storage domain mount paths to "localhost:<name>". The only thing that has prevented me from making this change is that my environment is live with running VM's. Please let me know if you go this route.
Thank you,
Tim Macy