[Conf] Make logging type and filename env-overridable

[Conf] Make logging type and filename env-overridable

Template the baseline logging block so deployments can switch logging
without patching conf/rspamd.conf:

    type = "{= env.LOG_TYPE|default('file') =}";
    filename = "{= env.LOG_FILE|default('$LOGDIR/rspamd.log') =}";

With no RSPAMD_LOG_TYPE/RSPAMD_LOG_FILE set the values render to the
previous hardcoded defaults (file, $LOGDIR/rspamd.log), so stock
installs are unchanged. A container can now set RSPAMD_LOG_TYPE=console
to log to stdout. Mirrors the env-template style introduced for the
worker bind_socket lines.