plugins: disable support for config dir

When removing and installing again the plugin all configuration files will be removed in `config/pluginname` dir.
This is bad as users may have set and added specific configuration files.

During an install, if we detect already existing files in `config/pluginname` directory, we simply copy the new file to the same dir but we append a `.new` at the end.

Related to  #5064.

(cherry picked from commit 5da028f)
(cherry picked from commit 4cb1f95)
[SNAPSHOT] Add repository validation

Fixes #7096
Add Upgrade API

This commit does the following:
* Add the new API at the rest layer, being backed by the optimize API
  with upgrade flag, and segments api to find upgrade status.
* Add `upgrade` flag to optimize API, and deprecate `force` flag (will
  remove in master)
* Add test for both synchronous and async upgrade

closes #7884
closes #7922
Remove deprecations from master (follow up to #7922)
Fix missed places referencing optimize force flag from removal in commit 1ae8195.
Docs: fix formatting in snapshots documentation