Separate chain.producerPrivKey from config.yaml

[raullenchai]

What would you like to be added:

• separate producerPrivKey from config.yaml
• have a new yaml file, e.g., privkey.yaml, for this field

Why is this needed: For now, chain.producerPrivKey is part of config.yaml and creates two problems

• privacy concerns: ideally config.yaml should only contain public info
• upgrade concerns: when upgrading a node, it is easy to pull/copy a new config.yaml to overwrite the existent ones which cause loss of privkey info, and gets the node into trouble.

[raullenchai]

Dup of https://github.com/iotexproject/iotex-core/issues/3600