Add default jenkins version to Makefile of master image

When building the jenkins-docker/master image manually using the
provided Makefile the build failed, because the JENKINS_WAR_VER
and JENKINS_WAR_SHA were not set.

To allow an easier manual build this change sets a default Jenkins
version (2.25).

Change-Id: Ieeee8ffbbc722be508c5caa6ac47943b6dc9249f
diff --git a/jenkins-docker/master/Makefile b/jenkins-docker/master/Makefile
index dfc1662..7b0d042 100644
--- a/jenkins-docker/master/Makefile
+++ b/jenkins-docker/master/Makefile
@@ -7,6 +7,8 @@
 OAUTH_SECRET=secret
 JENKINS_API_USER=user
 JENKINS_API_PASSWORD=pass
+JENKINS_WAR_VER=2.25
+JENKINS_WAR_SHA=1c4a750b7d7102c5fc9b78550fd3dcc93d7374da
 
 # Targets