[Tools] Minor fixes in release script

- Introduced required RELEASE_BRANCH environment variable
- Some hardcoded binary calls (gpg, sha512sum, md5sum) have been
  replaced by environment variables (GPG, SHASUM, MD5SUM) and added
  "" argument for 'sed -i' calls for compatability with OS X
- Removed unused environment variables
- Directly call ssh/scp instead of going through sshpass