tiup playground on low disk machine cannot schedule replicas

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

I'm using tiup playground on a machine with 466Gi total space and 34Gi available space. I created the cluster with 3 kv nodes:

tiup playground nightly --kv=3 --tiflash=0

2. What did you expect to see?

With 3 kv nodes, and max-replicas = 3, I expected that any tables I create would have 3 replicas.

3. What did you see instead?

Because PD detects low space on the tikv nodes, it only creates 1 out of the 3 expected replicas (see https://github.com/tikv/pd/issues/3444).

I think tiup should possibly set the pd option schedule.low-space-ratio = 0.99 or similar so that we have a better chance of the cluster operating in an environment with low available space.

4. What version of TiUP are you using (tiup --version)?

v1.3.2 tiup
Go Version: go1.13
Git Branch: release-1.3
GitHash: 2d88460

Start with the tiup playground command and the PD configuration it generates, then review how schedule.low-space-ratio is passed to PD. Reproduce the low-disk three-KV-node setup if possible and verify that the resulting cluster can schedule the expected replicas without changing behavior for normal environments.