Some components like neutron-lib builds its own sub-enforcer which
enforces policy rules partially. However even these enforcer may load
the full policy rules in the file and this causes a lot of warnings
about "undefined rules".
This introduces a new flag so that users can disable undefined check,
when they know the undefined rules are "expected".
Note that the flag is not formally exposed, because we don't know if
this requirement is common. If we find similar problems with different
components then we may add an argument to __init__ .
Related-Bug: #2048198
Change-Id: Ibb4e8e877640e8488aaffb40560e930b0cbfcbce
973498106f