cloud-init support

[shaunco]

We have an increasing number of customers deploying balenaOS virtual appliances on VMWare, Nutanix, or Azure and we currently provide them a pre-configured OVA. When delivering a few, it isn't too bad to edit the config.json file in the boot partition and export a new OVA, but for larger deployments it would be great for balenaOS to support cloud-init to at least obtain network config (IP/mask, gateway, DNS, hostname, NTP, etc) and the remainder of config.json settings via vendordata for things like balena hostnames, provisioning keys, redsocks config, CA roots, etc.

[ab77]

@shaunco we use this. It preloads a stub image that pulls cloud-init config from provider metadata.