sudorule: Fix runas with external users and groups.

When setting 'runasuser' or 'runasgroup' for a sudorule, either IPA or
external users and groups can be used, but only IPA users and groups
were being searched for when modifying the attributes, making this task
not idempotent if an external group or user was used..

This patch fixes this issue by comparing users and groups to the IPA
and external setting.

The IPA CLI commands are slightly confusing, as the sudorule-add and
sudorule-mod display separate options for internal and external users
and groups, but these options are deprecated and do not work anymore,
in favor of sudorule-add-runasuser and sudorule-add-runasgroup, which
don't diferentiate between internal and external users, from the CLI
user perspective.