ipatests: fix bind service name
With the commit 721435cf7f2ed41fe807c34022fed31c792b4497
the service name for bind is now 'named' instead of
'named-pkcs11' on fedora. The ipa-healthcheck test was hardcoding
the service name but it should instead use the name stored in
knownservices.named.systemd_name as it varies depending on
the OS.
Fixes: https://pagure.io/freeipa/issue/8482
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>