Format timestamps using ISO 8601 formatting (e.g. "2018-02-28T12:34:56Z")

* Z is the zone designator for the zero UTC offset.
* T is the delimiter used to separate date and time.

This makes it clear for the end-user that the date/time logged is
specified in UTC and not in the local time zone.

test/functional/combine_logs.py

@@ -13,7 +13,7 @@ import re
 import sys
 
 # Matches on the date format at the start of the log event
-TIMESTAMP_PATTERN = re.compile(r"^\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}\.\d{6}")
+TIMESTAMP_PATTERN = re.compile(r"^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{6}Z")
 
 LogEvent = namedtuple('LogEvent', ['timestamp', 'source', 'event'])