core: Use the latest OVF_STORE files

Rotate over the OVF_STORE disks and retrieve the entities from the best
match of the OVF_STORE disk until succeeded.
Also register those disks as OVF_STORE disks, so it will be updated
every time there will be any change in the entities' OVF.

Change-Id: I8252281f295df49a28c3c131d9a98fef9af8f35e
Bug-Url: https://bugzilla.redhat.com/1138114
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>