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.
The plugin script parses command-line options looking for Java system
properties and extracts these arguments to pass to the java command when
starting the JVM. Since elasticsearch-plugin allows arbitrary user
arguments to the JVM via ES_JAVA_OPTS, this parsing is unnecessary. This
commit removes this unnecessary
Relates #18207
Jason Tedor
Tanguy Leroux
Jim Ferenczi
Clinton Gormley