Fix pod info dashboard

Currently kas' dashboard shows kas, mailroom and sidekiq in "Active version" panel: https://dashboards.gitlab.net/d/kas-pod/kas-pod-info?orgId=1

Mailroom dashboard has the same problem: https://dashboards.gitlab.net/d/mailroom-pod/mailroom-pod-info?orgId=1

This is because pods are not filtered by pod name and other templated variables. This MR fixes that. It also cleans up some duplication and fixes a few more things - see individual commits.

./test-dashboard.sh ./kas/pod.dashboard.jsonnet
Installed https://dashboards.gitlab.net/dashboard/snapshot/iCm4lgflv8eHP9SG9O6kn54jG9mnwEiS - kas: Pod Info

Relates to gitlab-org/cluster-integration/gitlab-agent#149.