Serialize `last_controller` safely

Don't rises exception when a user tries to get default deployment recipe
for a cluster without controllers. Just send him next line

    "last_controller": null

with response.

Change-Id: Ie5a47c2cf0e72017a125ca45976de83447c2a8c2
Closes-Bug: #1296861
Merge "Serialize `last_controller` safely"