Config parsing: fail on unknown fields and print them

[Al2Klimov]

Useful against config validation or runtime failures caused by wrong field spelling or YAML indentation.

➜  icingadb git:(yaml.DisallowUnknownField) ✗ go run ./cmd/icingadb-migrate -c i2i.yml -t `mktemp -d`
can't parse config file: [1:1] unknown field "Icinga2"
>  1 | Icinga2:
       ^
   2 |   env: da39a3ee5e6b4b0d3255bfef95601890afd80709
   3 | ido:
   4 |   type: mysql
   5 |
exit status 2

refs #574

TODO

• review (non-author)
• unit test that particular case in FromYAMLFile() (author)

[julianbrost]

Looks fine in general, just a few things:

1. I wouldn't really consider this a bug (fix), just a more helpful error message.
2. Shouldn't this fix the linked issue?
3. There are some places that are just a map[string]... to the YAML parser, so there you can still miss typos. For example:

   logging:
     options:
       something-that-might-have-a-typo: debug

   Nothing that should stop this PR, but it might be worth a look if we can add something to the Validate() functions that checks that there are no unexpected keys there.

[Al2Klimov]

Regarding the issue, this is only half of the rent. But this is already an improvement on its own.

[julianbrost]

review (non-author)

The code changes look fine to me.