device-types api: allow to get/set health-checks
device-types api: allow to get/set health-checks

Also make the code consitent by always checking that the corresponding
device-type exists.

Change-Id: I1ab8c3cc038cffd9b3747875e490bdee64a24168

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server