Handle channels in release script

There are currently too many hard strings in the tools/release.sh script related to "release" or "beta".
The idea is to review them to be customizable with arguments.
The goal is to be able to generate alpha installers and deploy them via the Drive-beta job without creating another release script file.

Let's say that:

• VERSION_GIT is the output of the git describe --all --match="release-*" command, eg tags/release-4.0.0-32-g3175b139.
• VERSION_FULL is a version number base on VERSION_GIT in the form X.Y.Z.N. For instance 4.0.0.32.
• VERSION is a version number based on VERSION_FULL in the form X.Y.Z. For instance 4.0.0.

In alpha release, the script will have to do more things:

• change the version in nxdrive/__init__.py with VERSION_FULL.
• update the date in docs/changes/VERSION.md
• create the tag in the form alpha-VERSION_FULL.