Allow _meta.description to be customized

[fgierlinger]

When creating custom field definitions, the value for the field "_meta.description" is hard coded in the builder. It would be nice if this value could be customised. Especially when using subset files, this description is not accurate.

ecs/scripts/generators/es_template.py

Lines 215 to 217 in d76ae49

	template['_meta'] = {
	"ecs_version": ecs_version,
	"description": "Sample composable template that includes all ECS fields"