restapi: fix lack of errorTranslator mock

Fix in REST test infrastucture the lack of mocking for
error translator in setupCreationExpectations method.
Before the fix, passing to this method cando=false
or execute=false caused NullPointerException during
the test execution because the error translator was not mocked.

Change-Id: Ibc315c1baf2e85005bde7b637e9fff870ef66aa4
Signed-off-by: Alissa Bonas <abonas@redhat.com>