[docker-sonic-mgmt]: Upgrade Tgen API from 0.0.42 to 0.0.70 (sonic-net#6275)

Tgen API 0.0.42 has many problems. We have fixed them in 0.0.70.

Author: baiwei0427

dockers/docker-sonic-mgmt/Dockerfile.j2

@@ -40,7 +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 \
+                ixnetwork-open-traffic-generator==0.0.70 \
                 jinja2==2.7.2 \
                 jsonpatch \
                 lxml \