Fix breaking changes changes made in 1.12 and leftovers

[roman-vynar]

Fix breaking changes changes made in 1.12 and leftovers:

• http_timeout: 10 became http_timeout: "10s" in the samples, otherwise it will be nanoseconds and impossible to do Google auth etc. Nothing mentioned in the changelog.
• token_db config string became level_token_db.path in the samples. Nothing mentioned in the changelog.
• Fixed config options checks when level_token_db.path was set but docker_auth still complaining it is not.
• Replace deprecated ioutil.ReadAll > io.ReadAll