cmd: Use caddy.Log() instead of fmt.Printf for config adapter warnings

[francislavoie]

Lines in the logs like this:

2021/03/22 06:11:28.733 INFO    using adjacent Caddyfile
[WARNING][caddyfile] Caddyfile:2: input is not formatted with 'caddy fmt'

Now become:

2021/03/22 06:16:08.734 INFO    using adjacent Caddyfile
2021/03/22 06:16:08.735 WARN    input is not formatted with 'caddy fmt' {"adapter": "caddyfile", "file": "Caddyfile", "line": 2}

Just a bit nicer.

[mholt]

Thanks!