lxd-imagebuilder GPG tests should be updated

[simondeziel]

https://github.com/canonical/lxd-imagebuilder uses an old and expired key that belongs to an individual. The key in question is fetched from Ubuntu key server.

Assuming lxd-imagebuilder supports local GPG key, a better way would be to dynamically generate a key for the test or create a permanent one not tied to any individual and save it in the git repo to avoid the dependency on the external key server.

[tomponline]

@MusicDin do you mind taking a look at this too?