Don't use admin cert during KRA installation
KRA installation currently imports the admin cert. FreeIPA does not
track this cert and it may be expired, causing installation to fail.
Do not import the existing admin cert, and discard the new admin
cert that gets created during KRA installation.
Part of: https://pagure.io/freeipa/issue/7287
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>