fix #1423: add timezone config option

fix #1423: add timezone config option

optionally enabled.

adds the 'timezone' option to config, which takes a valid timezone (i.e.
Europe/Vienna) to set in the container.

if nothing is selected, then it will show as 'container managed' in
GUI, and nothing will be done.

if set to 'host', the /etc/localtime symlink from the host node will be
cached and set in the container rootfs.

Signed-off-by: Oguz Bektas <o.bektas@proxmox.com>