timer_management: Get correct UTC and timezone time

1. Both 'date -u +%s' and 'time.time()' will get localtime(timezone time)
   instead of UTC time.
2. 'time.mktime()' output is platform-dependent, and affected by daylight
   and timezone, which means translate timestr on different boxes may have
   different results. Here fetch both UTC and timezone time in the same
   format and translate on the same box to get relative gap.
3. For 'offset=timezone' setting, the host timezone should be set before
   the guest active.

Signed-off-by: Yanbing Du <ydu@redhat.com>