cloud-init: allow custom network/user data files via snippets

cloud-init: allow custom network/user data files via snippets

Adds the 'cicustom' option to specify either or both network and user
options as property strings. Their parameters are files in a snippets
storage (e.g. local:snippets/network.yaml). If one or both are specified
they are used instead of their respective generated configuration.
This allows the use of completely custom configurations and is also a
possible solution for bug #2068 by specifying a custom user file that
contains package_upgrade: false.

Tested with Ubuntu 18.10 and cloud-init 18.4.7

Signed-off-by: David Limbeck <d.limbeck@proxmox.com>