Slight refactoring of machine script generation.

[tasdomas]

Use embedded template instead of format string. Add test. Remove use of map pointer - golang maps are already pointers to a maps structure and can be nil.

[0x2b3bfa0]

Have you considered using github.com/sebdah/goldie for template tests, like in other places of this project? E.g.

https://github.com/iterative/terraform-provider-iterative/blob/142cd5b8b1aba9b0deb746b9fc5cfe3b6fecdd39/iterative/testdata/script_template_cloud_aws.golden#L1

[0x2b3bfa0]

Merging with AWS red because of capacity issues; works as expected.