[docker-base-stretch]: install rsyslog from stretch-backports (#5410)

Install a newer version of rsyslog from stretch-backports to support -iNONE

Previous backport from master use -iNONE option which is only
available after v8.32.0

Signed-off-by: Guohan Lu <lguohan@gmail.com>

dockers/docker-base-stretch/Dockerfile.j2

@@ -45,7 +45,6 @@ RUN apt-get update &&        \
         procps               \
         python               \
         python-pip           \
-        rsyslog              \
         vim-tiny             \
 # Install dependencies of supervisor
         python-pkg-resources \
@@ -66,6 +65,9 @@ RUN apt-get update &&        \
 # for processing json files in bash environment
         jq
 
+# Install a newer version of rsyslog from stretch-backports to support -iNONE
+RUN apt-get -y -t stretch-backports install rsyslog
+
 # For templating
 RUN pip install j2cli