update the monitoring stack for cdk

[kwmonroe]

• bump charm revs
• use bionic if possible
  • cant do it for subordinates yet because k8s principals are xenial
• add the telegraf dashboard back
  • recent grafana update wont load a dashboard if a metric is missing, which is the case for the default grafana/Telegraf2.json dash. Use a known good dashboard so users have something to build off of: https://code.launchpad.net/~chris.sanders/grafana-charm/+git/grafana-charm/+merge/348122

Tested well with conjure-up 2.6.0 on Darwin and AWS.