Unable to override composable templates settings

[faudebert]

Description of the problem including expected versus actual behavior:

When running scripts/generator.py with --template-setting, generated/elasticsearch/template.json is partially overridden with provided custom values. That is, if specified, mappings are properly overridden but template settings part always remains to its default.

Steps to reproduce:

1. Check template-settings.json content (only index_patterns here for brevity)

$ grep index_patterns usage-example/fields/template-settings.json
  "index_patterns": ["acme-weblogs-*"],

2. Run generator.py:

$ python scripts/generator.py --ref v1.6.0 \
  --subset            usage-example/fields/subset.yml \
  --include           usage-example/fields/custom/ \
  --out               usage-example/ \
  --template-settings usage-example/fields/template-settings.json \
  --mapping-settings  usage-example/fields/mapping-settings.json

3. Check template.json content (again, only index_patterns for brevity):

$ grep -A 2 index_patterns usage-example/generated/elasticsearch/template.json
  "index_patterns": [
    "try-ecs-*"
  ],

We are expecting "acme-weblogs-*".

Any additional context:

Reading the source, it looks defaults are hardcoded in save_composable_template() while its legacy equivalent is calling template_settings() which is reading the user-provided file.