Hi Guilio,
supposing that client.manila user is created on the external ceph server, the openstack node where manila-share is running should have following configuration snippet in /etc/ceph/ceph.conf:

[client.manila]
keyring = /etc/ceph/ceph.client.manila.keyring

So that manila ceph driver knows what keyring to use when connecting to the ceph server. This is different in compare to e.g. cinder/gnocchi where ceph keyring path is set directly in
the /etc/cinder/cinder.conf.

I don't have OC deployed with ceph-external templates so it might be that I'm wrong and the required config snippet is generated by ceph-external configuration (though "ManilaCephFSNativeCephFSAuthId" which sets manila ceph user is passed to manila puppet module only which does not update ceph.conf).

I'm also afraid that the default client.openstack is not sufficient ( maybe because it conflicts with cinder/gnocchi clients), if I use "openstack" user for manila driver, it fails with:

2016-12-01 09:22:02.788 232775 INFO manila.share.drivers.cephfs.cephfs_native [req-de3e597d-25c3-4be1-9394-868bdeedda7f - - - - -] [cephfs}] Ceph client found, connecting...
2016-12-01 09:22:02.817 232775 INFO ceph_volume_client [req-de3e597d-25c3-4be1-9394-868bdeedda7f - - - - -] evict clients with auth_name=openstack
2016-12-01 09:22:02.825 232775 INFO ceph_volume_client [req-de3e597d-25c3-4be1-9394-868bdeedda7f - - - - -] evict: joined all
2016-12-01 09:22:02.825 232775 ERROR ceph_volume_client [req-de3e597d-25c3-4be1-9394-868bdeedda7f - - - - -] Failed to evict client with auth_name=openstack from mds 0/14104: Error -1 ("") w
hile Sending evict to mds.14104
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager [req-de3e597d-25c3-4be1-9394-868bdeedda7f - - - - -] Error encountered during initialization of driver 'CephFSNativeDriver' on 'over
cloud-controller-0.localdomain@cephfsnative' host. Failed to evict client with auth_name=openstack from mds 0/14104: Error -1 ("") while Sending evict to mds.14104
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager Traceback (most recent call last):
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager   File "/usr/lib/python2.7/site-packages/manila/share/manager.py", line 269, in init_host
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager     self.driver.check_for_setup_error()
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager   File "/usr/lib/python2.7/site-packages/manila/share/drivers/cephfs/cephfs_native.py", line 88, in check_for_setup_error
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager     self.volume_client
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager   File "/usr/lib/python2.7/site-packages/manila/share/drivers/cephfs/cephfs_native.py", line 156, in volume_client
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager     self._volume_client.connect(premount_evict=premount_evict)
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager   File "/usr/lib/python2.7/site-packages/ceph_volume_client.py", line 467, in connect
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager     self.evict(premount_evict)
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager   File "/usr/lib/python2.7/site-packages/ceph_volume_client.py", line 421, in evict
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager     raise EvictionError(msg)
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager EvictionError: Failed to evict client with auth_name=openstack from mds 0/14104: Error -1 ("") while Sending evict to mds.14104
2016-12-01 09:22:02.826 232775 ERROR manila.share.manager