Validate websession-flatfile plugin using Gerrit CI

Change-Id: I58d239b0a6dd1ce4750901ed874bc527c40c8237

Jenkinsfile

diff --git a/Jenkinsfile b/Jenkinsfile
new file mode 100644
index 0000000..f5d8cba
--- /dev/null
+++ b/Jenkinsfile
@@ -0,0 +1,2 @@
++pluginPipeline(formatCheckId: 'gerritforge:plugins-websession-flatfile-code-style',
++               buildCheckId: 'gerritforge:plugins-websession-flatfile-build-test')