update: convert legacy grafana-server groupname early

update: convert legacy grafana-server groupname early

If the legacy name `grafana-server` is still being used when upgrading
from Nautilus to Pacific, the task that sets the fact `rolling_update`
to `true` doesn't run on the node(s) included in that group. Indeed the
play where we set this fact (`rolling_update`) only runs on the group
`monitoring_group_name | default('monitoring')`.
As a workaround, we can run earlier the task which converts the
`grafana-server` group name to `monitoring`.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1935554
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>