One branch to rule them all

[robaganrab]

Have a single master branch to hold the supported Debian versions.
Additional benefit would be to have security fixes incorporated in the images.

• master branch created.
• Subdirectories track the different releases. (jessie, stretch)
• CI config is building all images.
• Images are tagged with date as in the Debian images.
• Images are pushed in case it is a non-PR build. (to support scheduled builds)
• Tags without date (latest and jarokelo/debian:stretch) to be created and pushed.
• master branch is set as default.

As a second stage, enable for jessie. This is to prevent ruining the current images.
Third stage could be to trigger on upstream images publishing.