time: timezone should be set by backend based on provided region and city

Currently front-end need to know the GNU timezone name (e.g. `CET-1CEST,M3.5.0,M10.5.0/3`)
Which is bit harder to get and it makes no sense to have it
as a part of an API call. It can be derived from Region/Country

This commit alters backend to ignore provided `timezone` attribute,
makes it optional to `update_settings` and sets timezone properly
7 jobs for time-zone-out-of-api in 10 minutes and 11 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #496013
blackbox_mock_mqtt

00:03:07

passed #496014
blackbox_openwrt_mqtt

00:10:10

passed #496015
blackbox_openwrt_ubus

00:09:01

passed #496012
blackbox_openwrt_unix_socket

00:09:19

passed #496010
flake8

00:02:05

passed #496016
sample_module

00:02:53

passed #496011
unit_tests

00:02:52