Hello, just sharing the solution:
Effectively as highlighted by Alejandro the problem was related to permission but not the kind ones you can solve with chmod, it was related to apparmor policies.
After a deeper understanding of what apparmor does I created as root at all nodes (frontend doesn’t need it) a file in “/etc/apparmor.d/tunables/home.d/opennebula” with this content:
davidp@poc1:~$ cat /etc/apparmor.d/tunables/home.d/opennebula
@{HOMEDIRS}+=/var/lib/one/datastores
Followed by “# service apparmor restart”
After that the problems is gone 
As reported in this topic it would be helpful to include a call to apparmor stuff in the deployment guide for mounted datastores.
Thanks everybody!