dockers/docker-sonic-mgmt/Dockerfile.js: Add keysight ixnetwork-open-… (

sonic-net#5762)

*Why I did it
Provide access to the ixnetwork-open-traffic-generator pypi package

*How I did it
Added a pip install entry for the package in the Dockerfile.j2

*How to verify it
pip show ixnetwork-open-traffic-generator

*Description for the changelog
Install of ixnetwork-open-traffic-generator pypi package is required for proposed rdma tests.

dockers/docker-sonic-mgmt/Dockerfile.j2

@@ -40,6 +40,7 @@ RUN pip install cffi==1.10.0 \
                 ipaddr \
                 ipython==5.4.1 \
                 ixnetwork-restpy==1.0.52 \
+                ixnetwork-open-traffic-generator==0.0.42 \
                 jinja2==2.7.2 \
                 jsonpatch \
                 lxml \