Fix Jenkins CI master docker when creating ~/.ssh

The .ssh directory may already exist on the docker image.
Make its creation more resilient using the '-p'.

Change-Id: I34661923474a819a046a7ab78af5401dda75d26c
diff --git a/jenkins-docker/master/Dockerfile b/jenkins-docker/master/Dockerfile
index f2954dd..56a057e 100644
--- a/jenkins-docker/master/Dockerfile
+++ b/jenkins-docker/master/Dockerfile
@@ -47,7 +47,7 @@
 COPY config.xml $JENKINS_REF
 COPY jenkins.plugins.logstash.LogstashInstallation.xml $JENKINS_REF
 
-RUN mkdir $JENKINS_REF/.ssh && \
+RUN mkdir -p $JENKINS_REF/.ssh && \
     chown -R jenkins:jenkins $JENKINS_REF/.ssh && \
     chmod -R 700 $JENKINS_REF/.ssh