With the unified release process across the elastic stack, download
links for all products are changing. This change updates docs referring
to the old download and packages urls.
Note that this change also updates the plugin installation command as
the url for downloads is being changed to be consistent with that for
packages (both plural).
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.
