awsmp public-offer instance-type-template creates the old style instance_type.csv

**Describe the bug**
After migrating instance types to the awmsp config file the instance-type-template was never updated to produce a template in the new config format

**To Reproduce**
run `awsmp public-offer instance-type-template`

**Expected behavior**
`awsmp public-offer instance-type-template` produces a minimal yaml file with an instance_type template

**Additional context**
NA

Start with the `awsmp public-offer instance-type-template` entry point and reproduce the current CSV output. Trace the command to the template-generation code, then verify that it emits a minimal YAML file containing an instance_type template as described.

Written by the indexing model from the issue text.