[#588] Discard NO_IMEI value for imei property

* When searching for survey assignments, we want to search first using
  `imei` but WiFi only devices can only be identified with MAC address
  (stored as `phoneNumber`)
Merge pull request #589 from akvo/issue/588

[#588] Discard NO_IMEI value for imei property
[#590] Release Notes v1.6.17.1
Merge branch 'release/1.6.17'