Clean up config environment variables for server and agent

[marians]

close #209

The goal here is to make consistent use of configuration environment variables prefixed WOODPECKER_. Where several variants existed, this PR aims to remove all but one option, leaving the most explicit.

This PR only changes server and agent code, but not documentation, in order to keep the PR digestible. Once we have consensus that this is correct, I'll change docs accordingly.

User (rather: admin) facing changes in this PR:

• In general, support for all server and agent config environment variables (env vars) starting with DRONE_ is removed. The according WOODPECKER_* variables must be used instead.
• The env var WOODPECKER_HOST replaces DRONE_HOST, and DRONE_SERVER_HOST.
• The env var WOODPECKER_AGENT_SECRET is used to configure the shared secret which agents use to authenticate against the server. It replaces WOODPECKER_SECRET, DRONE_SECRET, WOODPECKER_PASSWORD, DRONE_PASSWORD, and DRONE_AGENT_SECRET.
• The env var WOODPECKER_DATABASE_DRIVER replaces DRONE_DATABASE_DRIVER and DATABASE_DRIVER.
• The env var WOODPECKER_DATABASE_DATASOURCE replaces DRONE_DATABASE_DATASOURCE and DATABASE_CONFIG.

Todo

• cleanup cli flags
• add migration notes to docs
  • instead of DRONE_ use WOODPECKER_
  • renamed env-vars and their new names (WOODPECKER_AGENT_SECRET, WOODPECKER_SERVER_HOST, WOODPECKER_DATABASE_DRIVER, WOODPECKER_DATABASE_DATASOURCE)
• update docs (docker-compose samples, server config)
• update sample docker-compose

Wait for

• Simplify docker-compose samples #356

[marians]

TODO: As pointed out in #209 (comment) the XDG_ variable should not be touched here.

[marians]

I just reverted the removal of XDG_CACHE_HOME.

[marians]

@laszlocph This is ready for a review.

[6543]

got conflicts :O

[6543]

I'll review it soon :)

[marians]

Thanks for taking over!