Allow nodes to send `null` in disk model field

Sometimes our discovery agents collects invalid data and sends it to
Nailgun. Because of adding JsonSchema for validation node model, the
requests with partially invalid data will fail and agents will not
be able to register themself.

Closes-Bug: #1292010
Change-Id: I319b142628e29e6706df71f0336a4d3ea71ddc22