Added support for setting timezone

A setting named timezone can be added to installer-config.txt which
will set the timezone of the installed system. The value (default is
Etc/UTC) must be a valid timezone identifier (e.g. Europe/Ljubljana)
Added support for setting locale

A setting named locale can be added to installer-config.txt which will
set the locale of the installed system (env. variables LANG and LC_ALL
in /etc/default/locale). The value (default is C.UTF-8) must be a valid
locale identifier (e.g. en_US.UTF-8)
Added support for setting language

A setting named language can be added to installer-config.txt which
will set the language of the installed system (env. variables LANGUAGE
in /etc/default/locale). The value (default is empty and will not be
set) must be a valid language identifier (e.g. en_US:en)