Add '-E' CLI flag

[urso]

Add '-D' CLI flag for overwriting single config options via CLI. For example:

-D output.console.pretty=true enables console output with pretty printing
enabled.

[urso]

blocked by #1985

[urso]

config values overwritten from command line are remembered by config system. e.g. error message from new system test:

error unpacking config data: required 'object', but found 'string' in field 'output.console' (source:'command line flag')

[tsg]

Niiiiiice!

I'm not sure about the best flag to use for this. Elasticsearch replaced -D with -E so they don't get confused with the JVM options. I guess -E is from elasticsearch so that doesn't work. Perhaps -C would be an option from "config". In any case, it's hard to be consistent here :(.

@elastic/beats any other ideas?

[urso]

One of:

- D = define
- E = environment
- C = config
- S = set

Don't like -C, as it might get mistaken for -c by accident.

[tsg]

Then I'm +1 for -E to be consistent with Elasticsearch. We can pretend it's E for "environment" :)

[tsg]

@urso there's a test failure.

[ruflin]

The test would be a little bit nicer if you would run it first with the correct one to see if it works and then with the failing one ...

[urso]

what exactly would you look for? Just relying on exit_code doesn't tell me if configuration was overwritten correctly.

Would love to introduce another test showing -E did apply config correctly. Maybe introduce some special config setting for printing a custom message (like mockbeat.echo) ?

[ruflin]

Yes, but it would confirm that the config you set worked without the -E flag.

[ruflin]

LGTM. Left a minor comment.

[ruflin]

LGTM. Waiting for green.