Add default affinity for tidb if bin-log enabled

[Yisaer]

Feature Request

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

pingcap/docs-cn#2073

tidb is recomended to deploy into the same node with pump. Currently the affinity in our helm chart values for tidb is empty. I think it's better to set affnitiy for tidb to stay in the node with pump by default if bin-log enabled.

Teachability, Documentation, Adoption, Migration Strategy:

https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#affinity-and-anti-affinity

[tennix]

The affinity should be set on TiDB instead of Pump. If binlog is enabled, the tidb-server should be scheduled on a node that has pump pod.

[Yisaer]

The affinity should be set on TiDB instead of Pump. If binlog is enabled, the tidb-server should be scheduled on a node that has pump pod.

Updated.

[tennix]

I don't think we should set it in values.yaml. This logic is complicated in helm. Instead we should add documentation for this.

[Yisaer]

Maybe we could recommend the affinity both in the document and the note in the values.yaml if the user want to enable the binlog? @tennix