enhance ttl config api output

[lhy1024]

Enhancement Task

1. I run a cluster with TTL config and I cannot modify it directly.

» config set merge-schedule-limit 16
Failed to set config: [400] "need to clean up TTL first for schedule.merge-schedule-limit"

2. I try to call API to delete it. Although it shows "config is updated", it is not changed.

curl -X POST -H "Content-Type: application/json" -d '{"schedule.merge-schedule-limit": 16}' "http://<ip_address>:<port>/pd/api/v1/config?ttlSecond=0"

[lhy1024]

actually, we only delete ttl config when post with ?ttlSecond=0 and we need to modify config with pd-ctl again.