Fix quickstart

[mneverov]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Fixes: #2986

Changes in quickstart.sh:

• MINIO_ENABLED is disabled by default
• S3_ENDPOINT is empty by default and filled in only when minio is enabled, hence thanos does not run store
• thanos arguments are parsed correctly (no empty arguments OBJSTORECFG, and STORES)
• thanos arguments are dumped in console on parse error. NOTE please object if you think it might be a security issue - i'll remove it

I didn't add anything to the changelog, since the last ~10 commits to the quickstart.sh are not reflected there.

Verification

Verified manually by running PROMETHEUS_EXECUTABLE=~/go/src/github.com/prometheus/prometheus/prometheus THANOS_EXECUTABLE=~/go/bin/thanos MINIO_ENABLED="" ./scripts/quickstart.sh.

log is attached.
log.txt

Note, that there are still errors level=error name=query-0 ts=2020-08-05T13:14:02.164403579Z caller=logger.go:22 msg="failed to flush Jaeger spans to server: write udp 127.0.0.1:54187->127.0.0.1:6831: write: connection refused", that might need investigation

[GiedriusS]

Maybe @kakkoyun you could help out in reviewing this? 🤗

[kakkoyun]

LGTM. Awesome that you have caught this, thanks a lot.
This is mostly my misinterpretation of linter warnings in #2969.
Tried and works as expected.