Print usage on stdout, and only when asked with -help.

[pstibrany]

This is small quality-of-life improvement to improve handling invalid flags and -help.

Invalid flags no longer trigger printing of all available flags (but is still printed to stderr):

$ ./cortex -asdasdsa > /dev/null
flag provided but not defined: -asdasdsa
Run with -help to get list of available parameters

Available flags are printed to stdout, so that extra redirection of stderr to stdout is not required to use with grep or less or other tools.

$ ./cortex -help | grep compactor.ring.consul
  -compactor.ring.consul.acl-token string
  -compactor.ring.consul.client-timeout duration
  -compactor.ring.consul.consistent-reads
  -compactor.ring.consul.hostname string
  -compactor.ring.consul.watch-burst-size int
  -compactor.ring.consul.watch-rate-limit float

Inspired by recent Slack discussion and similar change in Loki (grafana/loki#1396).

Checklist

• Tests updated
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[pracucci]

Looks a nice addition. I would have found it useful in few occasions in the past.