MLT Query/API: fix `minimum_should_match` bwc

Rounded to the nearest int allows to avoid issues in which (int) (0.59f * 100)
= 58, instead of 59%.
Changed the root rest endpoint ('/') to use cluster service

Instead of issuing a redundant cluster state request.

Closes #7899
Snapshot/Restore: Allow custom metadata to specify whether or not it should be in a snapshot

Before this change all persistent custom metadata is stored as part of snapshot. It requires us to remove repositories metadata later during recovery process. This change allows custom metadata to specify whether or not it should be stored as part of a snapshot.

  Fixes #7900
PluginManager: Fix config path extraction from plugin handle

The PluginManager had a subtle bug in case the config directory was not in the
es home directory - which is always true in case of packaging.

This fixes the plugin manager, so that when specifying a path.home and a
path.conf variable on the commandline, the plugin manager acts
appropriately.