[sonic-slave-stretch]: Adding package xmltodict. (sonic-net#25)

This package is needed to run sonic-yang-mgmt build test cases.

sonic-slave-stretch/Dockerfile.j2

@@ -355,6 +355,7 @@ RUN pip install setuptools==40.8.0
 # For sonic_yang_mgmt build
 RUN pip install ijson
 RUN pip install jsondiff
+RUN pip install xmltodict
 
 # Install dependencies for isc-dhcp-relay build
 RUN apt-get -y build-dep isc-dhcp