log format should be configurable
Currently, if log is stored to file by Kea all is ok. Log looks like that:
2019-06-12 02:35:45.834 INFO [kea-dhcp4.hosts/3024] HOSTS_BACKENDS_REGISTERED the following host backend types are available: mysql postgresql
But if Kea is run via systemd and logs are directed to stdout, captured and stored by systemd then logs look like that:
Jun 12 02:35:45 hmr-fedora-29-kea-srv-222549109 kea-dhcp4: 2019-06-12 02:35:45.832 INFO [kea-dhcp4.dhcp4/3024] DHCP4_STARTING Kea DHCPv4 server version 1.6.0-beta-git starting
ie. date, process name and process pid are duplicated.
User should be able to indicate expected log format.