Don't manage the nova uid/gid if nova_{user,group}_id is undef

Any user modification before standard system package creates the
user is wrong. Usecase of patch adding the user is also broken as
there are several other options: provide user before whole class
nova, have user id enforced by package or as most humans do accept
that in multi host enviroments there are 3 layers - user, group and
netgroup. In all cases nova puppet module should never touch the
user as it's provided by package on all supported systems, but for
backwards compatibility the management of user is disabled if
nova_group_id is undef or if nova_user_id is undef.

Closes-Bug: rhbz#1115946
Change-Id: If22b27f5fb78ba6821f4d6e8c275f8b80c70dd59
Merge "Don't manage the nova uid/gid if nova_{user,group}_id is undef"