alerts: Replace ntp with timex metrics

A local ntp server is optional, while timex is in kernel and always
present. Some ntp daemons like chrony can also only be run in a client
mode, which synchronizes the kernel but doesn't expose an ntp server.
Using the timex metrics is a more reliable approach.

jsonnet/prometheus-adapter: add metrics to default user-facing roles

Currently, only cluster wide admins have the permissions to view
metrics resources. This fixes it by adding a read-only cluster role
which includes aggregation labels to synthesize permission rules for
standard user-facing roles according to [1].

Note that only the "pods" resource is granted as reading "nodes"
metrics requires a cluster wide permission.

[1] https://kubernetes.io/docs/reference/access-authn-authz/rbac/#user-facing-roles

Update GitHub issue templates

Link fixes

README fixes, Dockerfile bump

update README.md

Update to Alertmanager v0.16.2

Remove node interface down

This alert is not specific enough on the interfaces it selects which in
turn makes it too noisy.

Add missing adapter to build snippet

Remove grafana-image

It's long obsolete

These metrics can still be valuable to troubleshoot components running
in cgroups on the host, outside the scope of Kubernetes pods.

Fix go get jsonnet command.

cAdvisor exposes metrics for each cgroup hierachy step, and containers
are part of the respective pod's hierarchy, causing double accounting
when not filtered appropriately.

Fix node:node_disk_saturation rules to return value in seconds