Improve gerrit_dash_creator.cmd.creator

* use configparser instead of self written parsing
  (add new method read_dashboard_file)

* remove methods get_title and get_sections

* add requirement six (ConfigParser in Python2,
  configparser in Python3)

* remove import re and fileinput

* check if dashboard file exist and is readable before
  trying to parse it (new imports os and os.path)

* slighly change the output to be more informative

* add new parameter in section dashboard to make the
  base URL configurable

* check if necessary options in the used dashboard
  file are available

* improve exception handling and error output

* rename generate_url to generate_dashboard_url

* use correct exit codes (correct return values in the
  main method)

* rename internal argument name dash to dashboard_file

* add missing doc strings

Change-Id: Ifcaf20f7495b5619db30ac0046909b334672f756