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[3024]: 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.

Edited Jun 12, 2019 by Michal Nowikowski

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