lxc-opensuse template now understands release arguments

Signed-off-by: Johannes Kastl <git@ojkastl.de>
lxc-opensuse.in: Added explanation on how to use the template

Signed-off-by: Johannes Kastl <git@ojkastl.de>
lxc-opensuse.in: Check if given argument is a valid release

Signed-off-by: Johannes Kastl <git@ojkastl.de>