Rename obsolete "parity" EthereumJSONRPC.Variant to "nethermind"

[vbaranov]

Motivation

• Obsolete "parity" EthereumJSONRPC.Variant
• Repeated peace of code in the config of Blockscout DBs.

Changelog

• replace "parity" EthereumJSONRPC.Variant with "nethermind" (backward compatibility: "parity" Ethereum JSON RPC Variant still be valid)
• Wrap repeating pieces of DBs config into public functions of DB config helper.

Docs update in blockscout/docs#68

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools