monitoring: Set the Grafana hostname in the inventory

Running the monitoring playbook currently fails:

$ ansible-playbook digital-ocean-k8s/monitoring.yaml --diff --check -i inventories/production/
…
TASK [monitoring : Deploy Grafana] **************************************
Saturday 11 November 2023  11:58:37 +0100 (0:00:06.898)       0:00:25.722
fatal: [monitoring]: FAILED! =>
  msg: |-
    The task includes an option with an undefined variable.
    The error was: 'hostname' is undefined

Set the hostname to match what has been already deployed.