This commit removes the ability to specify a custom plugins
path. Instead, the plugins path will always be a subdirectory called
"plugins" off of the home directory.
Today when parsing settings during bootstrap, we add a system property
for every Elasticsearch setting. Additionally, settings can be set via
system properties. This commit simplifies this situation.
- settings are no longer propogated to system properties
- system properties can not be used to set settings
- the "es." prefix on settings is no longer required (nor permitted)
- test logging has a dedicated system property (tests.logger.level)
Relates #18198
Clinton Gormley
Jason Tedor