[Fleet] Set default settings in component template instead of default template

[simitt]

Kibana version: 7.14.0

Describe the bug:
Fleet installs a custom component template and default component templates for settings and mappings if defined in the package. However, the created index_template always contains index settings for the refresh_interval, number_of_shards, and some others.
Settings defined in an index template can not be overwritten by component templates (see component template docs). Neither the default nor the custom component template can set settings such as number_of_shards, etc. at the moment.

It might be ok to fix some settings directly in the index_template, such as the lifecycle.name, but others such as number_of_shards and routing.* should be adjustable, at least by the package maintainers.

An open question is how this should be handled if nothing is specified in the package that would trigger creating a mapping and setting component template to be created.

Steps to reproduce:

1. Add some definition like the following to a package's datastream manifest:

elasticsearch:
  index_template:
    settings:
      number_of_replicas: 2

2. Start a local package registry containing the package changes
3. Start an environment that is consuming your local package registry and install the package and add it to the an agent policy
4. Define number_of_replicas: 5 in the @Custom component template
5. Ensure the data stream for the package is created (by ingesting some data, depending on the package)
6. Check the index settings (e.g. GET .ds-traces-apm-default-2021.07.14-000003/_settings) and see that the number_of_replicas is still set to 1 although a different value was defined for the package default and for the custom package.

Expected behavior:

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[simitt]

@jen-huang while I labeled this a bug for 7.14, it is not a problem for APM right now (will be in 7.15 though); please relabel if this better fits into 7.15.

[ruflin]

This also ties into #104620, we might not need most of these settings. Lets push this to 7.15.