Remove nodes only with correct environment ids

 * Fix in node unassignment handler's validator
 * Also fixed IvalidData error response formatting
 * Extended test for unassignment validation

 Closes-Bug: #1286091
 Closes-Bug: #1286036

Change-Id: Ie50954a5b043aa8141188bea345112232b6b7f57
Merge "Remove nodes only with correct environment ids"