cloud init: add warning to not install cloud-init on the host

cloud init: add warning to not install cloud-init on the host

From time to time, users install the cloud-init package on the host
by accident [0]. Since the other commands are intended to be run on
the host, add a warning about the one command that shouldn't.

Patch suggested by @Neobin on the community forum [1].

[0] https://forum.proxmox.com/threads/115530/post-499610
[1] https://forum.proxmox.com/threads/115530/post-499624

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>