Makes sure the GPG key fingerprint 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 makes the distribution of the GPG key
more secure an less likely to change even if 3rd party gains access to the
packages repository.

Docker-DCO-1.0-Signed-off-by: Jonas Pfenniger <zimbatm@zimbatm.com> (github: zimbatm)
Fixes key installation for other target urls in hack/install.sh

Docker-DCO-1.0-Signed-off-by: Jonas Pfenniger <zimbatm@zimbatm.com> (github: zimbatm)
Merge pull request #3361 from zimbatm/gpg-key-secure-best

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