Allow specifying custom buildspec during VM creation

[jimeh]

Under the hood the provider uses the build_from_spec API endpoint to
create a VM.

We should investigate if it's possible to provide custom buildspec in XML, JSON,
YAML, or HCL, while Terraform behaves normally throughout the rest of the VM
resource's lifecycle, as the spec is only relevant during creation.

Potentially a better approach might be to allow ways to customize specific parts
of the buildspec used during creation if using a full spec proves difficult.