ceph-facts: fix grafana group conversion

ceph-facts: fix grafana group conversion

The conversion fact task was only executed when the grafana_server_group_name
variable was explicitly set in the user configuration. If an user was using
the default value then the conversion wasn't executed.

This also adds back the default grafana_server_group_name value in case user
was using the default value and to avoid undefined variable error.

Instead of hardcoding the "monitoring" group name then we can reuse the
monitoring_group_name variable.

There's no need to override the monitoring_group_name variable, it's either
using the default value or the one defined by the user.

Finally removing the delegate_to statement on the add_host task since it's
always executed on the ansible controller.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1903732
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>