logs: consider configurable timezone defaulting to UTC

[iurii-ssv]

It's much faster and less error-prone to coordinate and debug logs with timestamps in the same timezone:

• when asking operators to provide logs targeting a particular time-frame
• browsing through several different log files written by different SSV nodes

hence I suggest we write all logs in UTC timezone (which is considered standard I think).

In case operators might prefer to not have UTC timezone (for whatever reason, I'm not sure why they would though) it might be a configurable toggle in config file (defaulting to UTC if not specified otherwise).