test_nfs.py: switch to master_3repl
    
    test_nfs.py historically used master_2repl_1client.
    Now that master_3client exists, switch to that as it allows removal
    of custom install/cleanup steps.
    
    Fixes: https://pagure.io/freeipa/issue/8027
    Signed-off-by: François Cami <fcami@redhat.com>
    Reviewed-By: Sergey Orlov <sorlov@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>