Makes sure the GPG key is inlined in the doc everywhere.

Instead of `curl $url | apt-get add -` use the ubuntu keyserver and a full
fingerprint to retreive the key.

This is a follow-up on #1644 and #3346 .

I'm also asking the question on Launchpad to see any consensus can be gained: https://answers.launchpad.net/ubuntu/+source/apt/+question/241343