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
Support stop provisioning mechanism

Change-Id: Id18f11e39f47b6b4569a795fa8cf915dea64121b
Implements: blueprint fuel-stop-deployment
Support stop provisioning mechanism

Change-Id: Id18f11e39f47b6b4569a795fa8cf915dea64121b
Implements: blueprint fuel-stop-provision
Interrupting provisioning - Nailgun part

Change-Id: If8dd1baae4295fc046a169afdfec966fe5db28db