Test fix, switch to plugin with less dependencies (elastic#13672)

Fixes an integration test that verifies the capabilities of CLI tool to install a not bundled plugin.
Move away from logstash-input-google_cloud_storage which depends indirectly to OS's package named shared-mime-info, which is not always available.

qa/integration/specs/cli/install_spec.rb

@@ -118,7 +118,7 @@ def gem_in_lock_file?(pattern, lock_file)
     end
 
     context "install non bundle plugin" do
-      let(:plugin_name) { "logstash-input-google_cloud_storage" }
+      let(:plugin_name) { "logstash-input-github" }
       let(:install_command) { "bin/logstash-plugin install" }
 
       after(:each) do