[Bug][ECS 1.7] Strip `index` attributes on `wildcard` mappings

[andrewstucki]

Description of the issue:

The 1.7 experimental wildcard mappings need to have the index attribute stripped from them. The one that was causing problems for me is here:

ecs/experimental/generated/beats/fields.ecs.yml

Lines 1154 to 1163 in 3edb6f2

	- name: stack_trace
	level: extended
	type: wildcard
	multi_fields:
	- name: text
	type: text
	norms: false
	default_field: false
	description: The stack trace of this error in plain text.
	index: true

Here's the error I'm getting from Elasticsearch:

Failed to parse mapping: unknown parameter [index] on mapper [stack_trace] of type [wildcard]