Warn on missing telegraf relations on the input side

The telegraf charm supports relating to various charms on the input side beyond the simple juju-info interface:

requires:
  amqp:
    interface: rabbitmq
    scope: container
  apache:
    interface: apache-website
    scope: container
  elasticsearch:
    interface: elasticsearch
    scope: container
  exec:
    interface: telegraf-exec
    scope: container
  haproxy:
    interface: statistics
    scope: container
  influxdb-api:
    interface: influxdb-api
  juju-info:
    interface: juju-info
    scope: container
  memcached:
    interface: memcache
    scope: container
  mongodb:
    interface: mongodb
    scope: container
  mysql:
    interface: mysql-root
  postgresql:
    interface: pgsql
  redis:
    interface: redis
    scope: container
  sentry:
    interface: sentry-metrics
    scope: container

We should warn whenever those relations are missing as we're missing the chance of generating more useful metrics.

Note that at least in some case both the juju-info and the more specific relation need to be present. See the telegraf readme for details.