Add documentation on cloud-init and qemu-guest-agent

[zdzielinski]

Fixes #280

Changes proposed in this pull request:

• Adds documentation on cloud-init and qemu-guest-agent as generally required pre-configuration steps for VM templates and existing disks.

[ovirt-infra]

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[zdzielinski]

I'm a bit rusty on Travis / GitHub, so I'll leave this alone for now. I'd be happy to set in a more "approved" workflow for this as I add more PR's in the future. I leverage this provider somewhat frequently, so I may be adding more as time goes along - if the maintainers think it would be useful.

[emesika]

Please replace tabs with spaces

[ghost]

Dear contributors,

We are making some large-scale changes to the Terraform provider which affect this pull request. Please read the announcement on the oVirt blog.

We are very sorry for the inconvenience and would like to hear your feedback on the community call on the 14th of October.

[ghost]

Dear contributor, we are closing this pull request as a preparation for the move to the new Terraform provider announced above. If you feel that this PR is still relevant please re-file it once we have made the move to the new provider.