[ceph] contextualization disk on rbd

If you use ceph as the only datastore (image) without having a classical shared system datastore and use contextualization you will struggle by using livemigration. The contextualization disk is created on the default system datastore on the target host (local). If you dont have a NFS or other shared file system, you cannot live migrate even if you have a distributed filesystem like ceph, which is shared on all nodes if configured properly.

The fix works and is tested for 4.8 and should also work for 4.10 and creates contextualization disks on rbd named "$VM_ID-context" and deletes the disk as soon the vm gets destroyed. You have to set CEPH_CONTEXT=true to your template context section and configure a custom system datastore using the ceph tm.

Regards,

Sebastian