add ability to skip installation of cloud-init and qemu-guest-agent

[csmart]

If we have a base image that already has cloud-init and qemu-guest-agent installed, there's no need to do that task. I've also noticed openSUSE also often fails this step even though they are already installed, when trying to update cache as it wants to update them or something. Add a way to skip these on a per-VM basis.

 Failed to cache repo (1).", "Warning: Skipping repository 'Update Repository (Non-Oss)' because of the above error.", "Some of the repositories have not been refreshed because of an error.", "Loading repository data...", "Reading installed packages...", "'cloud-init' is already installed.", "No update candidate for 'cloud-init-19.4-lp151.2.18.1.x86_64'. The highest available version is already installed.", "'qemu-guest-agent' is already installed.", "No update candidate for 'qemu-guest-agent-3.1.1.1-lp151.7.12.1.x86_64'. The highest available version is already installed.", "Resolving package dependencies...", "", "Nothing to do.", 

[tinsjourney]

I guess you can close this issue, if we define virt_infra_guest_deps to empty array, it won't install cloud-int and qemu-guest-agent.