Correct Java setting to pass proxy options to plugin install

It appears that `proxyHost` and `proxyPort` are not valid Java
properties; they should be prefixed with either `http` or `https`. There
is no code that the flags pass through in `plugin` that munges the flags
in any way, so they should be passed through as indicated in the Java
docs.

See:
https://docs.oracle.com/javase/7/docs/technotes/guides/net/proxies.html
https://docs.oracle.com/javase/8/docs/technotes/guides/net/proxies.html

docs/plugins/plugin-script.asciidoc

@@ -165,18 +165,18 @@ sudo bin/elasticsearch-plugin install analysis-icu --timeout 0
 
 To install a plugin via a proxy, you can pass the proxy details in with the
 Java settings `proxyHost` and `proxyPort`. On Unix based systems, these
-options can be set on the command line:
+options can be set on the command line for both http and https:
 
 [source,shell]
 -----------------------------------
-sudo ES_JAVA_OPTS="-DproxyHost=host_name -DproxyPort=port_number" bin/elasticsearch-plugin install mobz/elasticsearch-head
+sudo ES_JAVA_OPTS="-Dhttp.proxyHost=host_name -Dhttp.proxyPort=port_number -Dhttps.proxyHost=host_name -Dhttps.proxyPort=https_port_number" bin/elasticsearch-plugin install mobz/elasticsearch-head
 -----------------------------------
 
 On Windows, they need to be added to the `ES_JAVA_OPTS` environment variable:
 
 [source,shell]
 -----------------------------------
-set ES_JAVA_OPTS="-DproxyHost=host_name -DproxyPort=port_number"
+set ES_JAVA_OPTS="-Dhttp.proxyHost=host_name -Dhttp.proxyPort=port_number -Dhttps.proxyHost=host_name -Dhttps.proxyPort=https_port_number"
 bin/elasticsearch-plugin install analysis-icu
 -----------------------------------