add white-list option in gen-ext-timezones.py

:Release Notes:
New options -w/--no-guess/--white-list-only for gen-ext-timezones.py turns
on mode that generates only time-zones that have descriptions in file
src/uiTzInfo.json .

:Detailed Notes:
- added option -w/--no-guess/--white-list-only to limit set of
  time-zones to only know ones
- brought back Country name overriding to uiTzInfo.json

:Testing Performed:
verified contents generated for desktop build

:QA Notes:

:Issues Addressed:
[MFTEVENTFT-1397] Custom time zone doesn't work
[MFTEVENTFT-1241] Settings app shows unsupported time zone

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <nikolay.orliuk@lge.com>

Change-Id: I0fc272608a78621da5dec00ca24275b6c359ac56
Reviewed-on: https://g2g.lgsvl.com/6445
Reviewed-by: DCO Verification
Tested-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Reviewed-by: Alexandr Smoktal <alexandr.smoktal@lge.com>
Reviewed-by: Denys Sokolovskyi <denys.sokolovskyi@lge.com>
Reviewed-by: Hyungjoon Lee <hyungjoon.lee@lge.com>
Tested-by: Hyungjoon Lee <hyungjoon.lee@lge.com>
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Merge branch 'develop'