avoid using short GPG key id

[Nobuto Murata]

The short GPG key id is not considered as secure. It is used in landscape-scalable, landscape-dense and landscape-dense-maas bundle.

====
        landscape-server:
            charm: cs:trusty/landscape-server
            constraints: mem=2048
            to: lxc:0
            num_units: 1
            options:
                source: deb http://ppa.launchpad.net/landscape/15.10/ubuntu trusty main
                key: 4652B4E6
====

Instead, ppa:landscape/15.10 should be used in this case.