Jenkinsfile: Publish release artifacts for release branches.

Previously, the Jenkinsfile did not publish anything except from the
repositories: tarballs would not get copied to https://dl.jami.net for
the nighty and stable channels, and no commit nor tag would be
published for the release.

This change addresses these shortcomings with the following changes:

1. Tarballs are produced for the nightly and stable channels, and
organized in a sub-directory matching their channel name.
2. Release commits are made to their corresponding
channel (stable/nightly).
3. Stable releases are also tagged with 'YYYYmmdd.$day_commits_count.$commit_id'.

* Jenkinsfile (SSH_PRIVATE_KEY): Remove variable.
(RING_PUBLIC_KEY_FINGERPRINT): Rename to...
(JAMI_PUBLIC_KEY_FINGERPRINT): ... this.
(GIT_USER_EMAIL, SSH_CRED_ID): New variables.
(params.DEPLOY): Fix description.
(Checkout channel branch): New stage.
(Generate release tarball): Also commit and tag.
(Publish release artifacts): New stage to publish conditionally based on the
DEPLOY parameter and the selected channel.
(Sign & deploy packages): Use the 'sshagent' step to setup SSH access.
<--remote-ssh-identity-file>: Remove argument.
* scripts/deploy-packages.sh (package_snap): Simplify.

Change-Id: I9008ecc2a4ef9820dbc96e26c966ae72110d897d