Fix script to install what onos-user-key need in Docker debian:jessie image

- Install ssh package
- Create ~/.ssh directory

Change-Id: Id6e486fd63f178a6311d41b9216872ca48c61ad2
diff --git a/Dockerfile b/Dockerfile
index 8e3f0a1..420074c 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -19,7 +19,8 @@
 
 # Build ONOS
 WORKDIR /src
-RUN     apt-get update && apt-get install -y python less zip curl oracle-java8-installer oracle-java8-set-default && \
+RUN     apt-get update && \
+        apt-get install -y python less zip curl oracle-java8-installer oracle-java8-set-default ssh && \
         cd onos && \
         tools/build/onos-buck build onos && \
         cp buck-out/gen/tools/package/onos-package/onos.tar.gz /tmp/ && \