Fix, test, and document behavior for configuration loading

[neongreen]

• What happens if at startup, one rule (out of several) in one domain (out of several) can't be parsed?
• Same, but during config reloading, not during startup.
• What happens when configuration contains duplicate domains?
• What happens when a domain contains duplicate rules?
• What happens with YAML files containing several documents?
• What is the minimal accepted configuration file? Is it the same for Fencer and for lyft/ratelimit?

Also:

• (done in More faithful replication of lyft/ratelimit config loading rules: part 1 #46, not documented) Turns out that lyft/ratelimit loads all files, not just .yml files.
• (done in More faithful replication of lyft/ratelimit config loading rules: part 1 #46, not documented) And it loads them recursively.
• Would it look at .foo/bar.yaml if RUNTIME_IGNOREDOTFILES is enabled?
• RUNTIME_IGNOREDOTFILES in general is not tested by our testsuite by our testsuite, but should be.

Also:

• When lyft/ratelimit finds no configs in the dir, it responds to requests with OK.
• When lyft/ratelimit doesn't find the dir symlink, it responds to requests with OK.
• (partly done in Fix behavior for "rules not loaded" and add an integration test #35, not tested properly, not documented) When lyft/ratelimit finds the config dir but even one file is corrupted (broken YAML or correct YAML but broken field names), it responds to requests with ERROR.
• When lyft/ratelimit can't read a file (e.g. chmod 0-ed), other files are loaded correctly and it responds to requests with OK.

And more:

• Q: What happens to symlinks to files outside the config dir?
  A: I checked and they are loaded just fine.
• Q: Symlinks to directories outside the config dir?
  A: They are not followed.
• Q: Symlink cycles?
  A: They are detected ("Too many levels of symbolic links") and other files are loaded correctly.

[neongreen]

"What happens" might also include "lyft/ratelimit keeps working, but replies to all requests with ERROR".

[mdimjasevic]

I'm taking the first question: What happens if at startup, one rule (out of several) in one domain (out of several) can't be parsed?

[mdimjasevic]

I'm taking "What happens with YAML files containing several documents?".

[mdimjasevic]

Now I'm taking "What is the minimal accepted configuration file? Is it the same for Fencer and for lyft/ratelimit?"

[mdimjasevic]

@neongreen, what should I do with your questions in the "And more" section that have both a question and an answer?

[mdimjasevic]

@neongreen, what should I do with your questions in the "And more" section that have both a question and an answer?

@neongreen, just a gentle reminder about this question.

[mdimjasevic]

@neongreen, this is a yet another reminder about the question above.