Use strict teleport.yaml validation in warning mode

[awly]

Strict YAML validation catches the cases where a valid config key is
placed in the wrong location in the config. These errors were not
caught by the old validation.
The failure is always reported, but only fails startup when both old and
new validations fail. This will let the users fix their configs during
6.0 release and we will start enforcing it in 7.0.

Example:

auth_service:
  data_dir: "/foo" # this field must live under "teleport:", not "auth_service:"

Output:

$ teleport start -c teleport-invalid.yaml
ERRO             "Teleport configuration is invalid: yaml: unmarshal errors:\n  line 6: field data_dir not found in type config.Auth." config/fileconf.go:303
ERRO             This error will be enforced in the next Teleport release. config/fileconf.go:304
[AUTH]         Auth service 5.0.0-dev:v4.4.0-alpha.1-262-g307040886-dirty is starting on 0.0.0.0:3025.
... continues startup ...

Updates #5056

[a-palchikov]

lgtm except for the time.Sleep - is logging always configured to output to stderr so the user can actually see it?

[awly]

@a-palchikov my reason for the sleep is that most users don't configure/run teleport manually, but provision it via some tooling (ansible, k8s, etc). So they don't see the logs most of the time, unless something goes wrong.
A startup delay might nudge them to investigate and actually look at logs.

This is kind of speculative, so if others also thing the sleep is a bad idea, I'll remove it.

[klizhentas]

@awly can you fix the output to remove newlines? see in the output how the line is quoted

[awly]

@klizhentas that output is coming from yaml.UnmarshalStrict, not from our code.
Should I strings.Replace all the newlines in the returned error?

[klizhentas]

@awly yes, I would remove newlines from the output

[awly]

Removed newlines, output looks like:

$ teleport start -c teleport-invalid.yaml
ERRO             Teleport configuration is invalid: yaml: unmarshal errors:  line 8: field data_dir not found in type config.Auth. config/fileconf.go:306
ERRO             This error will be enforced in the next Teleport release. config/fileconf.go:307

[awly]

@klizhentas PTAL