Unit tests for objects.InstancePolicy + a fix

Tests for:
  objects.InstancePolicy.CheckParameterSyntax()
  objects.InstancePolicy.CheckDiskTemplates()
  objects.InstancePolicy.CheckISpecSyntax()

Instance policies with an empty disk-template list now are reported.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>