contrib: add cloud-init support for terraform vms (#8394)

* contrib: add cloud-init support for terraform vms

This change enables instance customization via cloud-init,
for example: additional CA certs, custom SSH access etc.

* contrib: update docs for terraform cloud-init

* contrib: disable yamllint in cloud-init

require-starting-space rule breaks cloud-init header

* contrib: documenation formatting

* yamllint: disable comments related checks

* docs: markdown formatting