Logstash-plugin generate test in the acceptance suite doesn't work

There is a few errors in theses examples:

1. The `File.directory?` check wont work because logstash is not a path
2. The install command won't work because the plugin is not build as a `*.gem` and the path is invalid.

This is a revert of https://github.com/elastic/logstash/pull/6879

```ruby
describe "on #{logstash.hostname}" do

GENERATE_TYPES = ["input", "filter", "codec", "output"]

GENERATE_TYPES.each do |type|
context "with type #{type}" do
it "successfully generate the plugin skeleton" do
command = logstash.run_command_in_path("bin/logstash-plugin generate --type #{type} --name qatest-generated")
expect(logstash).to File.directory?("logstash-#{type}-qatest-generated")
end
it "successfully install the plugin" do
command = logstash.run_command_in_path("bin/logstash-plugin install logstash-#{type}-qatest-generated")
expect(command).to install_successfully
expect(logstash).to have_installed?("logstash-#{type}-qatest-generated")
end
end
end
end
end

Start with the acceptance-suite example in the issue and inspect the bin/logstash-plugin generate and install commands it invokes. Reproduce each plugin type, then verify that generation checks the actual output and that installation succeeds for the generated plugin.

Written by the indexing model from the issue text.