Slightly abstract the daemon logfile lookup

The original LOG_<DAEMON_NAME> constants for daemon logfiles are gone.
In their place there is a DAEMONS_LOGFILES dict, indexed by daemon name.

This is a minor change with the objective to uniform most of the
daemon's main() functions code, which is very similar one to the other.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>