Added the ability to specify a key file to the release script.

if none is provided, it will work as it did until now - i.e. not use the `-i <key>` option (neither for scp nor for rsync)
on my machine/OS if I don't use the `-i` option, none of the keys (I have multiple) from `~/.ssh` are used. I have to use this explicit switch and specify a key in order to be able to use it.