Add tzdata back (#25433)

It appears that the base Docker images have removed tzdata.
We use tzdata in the debconf tests, so add it back.

test/utils/docker/ubuntu1204/Dockerfile

@@ -45,6 +45,7 @@ RUN apt-get update -y && \
     rubygems \
     subversion \
     sudo \
+    tzdata \
     unzip \
     xsltproc \
     zip \

test/utils/docker/ubuntu1404/Dockerfile

@@ -42,6 +42,7 @@ RUN apt-get update -y && \
     ruby \
     subversion \
     sudo \
+    tzdata \
     unzip \
     xsltproc \
     zip \

test/utils/docker/ubuntu1604/Dockerfile

@@ -46,6 +46,7 @@ RUN apt-get update -y && \
     ruby \
     subversion \
     sudo \
+    tzdata \
     unzip \
     virtualenv \
     xsltproc \

test/utils/docker/ubuntu1604py3/Dockerfile

@@ -42,6 +42,7 @@ RUN apt-get update -y && \
     ruby \
     subversion \
     sudo \
+    tzdata \
     unzip \
     virtualenv \
     xsltproc \