Add a possibility for ImageUpdateAutomation to reference GitRepository from another namespace

[ilya-git]

Please see more discussions in this issue:
#1017

The problem is that we would like to keep app-relevant stuff in it's own namespace, like e.g. team1, team2, etc., but since GitRepository itself is in the flux-system, everything else should be in the flux-system itself.

Maybe there is some other proper solution to this, but it would probably be a good idea to allow ImageUpdateAutomation to reference GitRepository from another namespace?

[stefanprodan]

Duplicate of: fluxcd/image-automation-controller#85