tools: add custom private key option

Add -i option for release.sh that allows users to specify
non-default private key for ssh and scp commands.
Change argument parsing to getopts.

PR-URL: https://github.com/nodejs/node/pull/14401


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>