[gitlab] Deploy deb pkg to apt repo

[olivielpeau]

Adds a deploy stage to our gitlab pipeline that releases the deb package to the unstable branch of the staging repo located at https://s3.amazonaws.com/apt-agent6.datad0g.com

Example of a gitlab pipeline that uses this code: https://gitlab.datad0g.com/datadog/datadog-agent/pipelines/22095 (without the master branch execution restriction)

Notes:

• This stage is executed only on the master branch
• The repo is signed with our latest staging GPG signing key
• Requires [version] Fix timestamp detection in version omnibus-ruby#28 for the release to the apt repo to work correctly
• Adds basic init scripts for upstart (legacy ubuntu systems) and systemd (recent debian and ubuntu systems) to the package to ease the installation/usage of the package.

FIXME:

• For now the deploy uses the docker image that's defined here and that is used on Jenkins to run all of our deploy jobs. We'll have to use our own Dockerfile and add it to datadog-agent-builders at some point
• Debian only
• Very basic init scripts (and I didn't want to spend time writing a sysvinit script), that run the agent as root