playground: support specify PD port

[pingyu]

Feature Request

Is your feature request related to a problem? Please describe:

I use TiKV as a key-value service, with PD port as service entry point.

• Scene 1
  On a shared server, if the 2379 is occupied by others, the port of PD is changed after every restart.
• Scene 2
  For some automatic integration test cases that need more than one cluster (e.g., backup & restore, change data capture), it is tedious to parse output of TiUP to acquire the PD port of the second cluster.

Describe the feature you'd like:

Support specify PD port in playground.

Why the featue is needed:

To be convenient for TiKV developers. The scenarios is very similar to db.port for TiDB users.

Describe alternatives you've considered:

Add an optional pd.port argument for tiup playground, and set Port of PDInstance to utils.MustGetFreePort(pd.host, pd.port).

Teachability, Documentation, Adoption, Migration Strategy: