[sonic-mgmt]: Support for pytest-html to control logs better (#4791)

The current stdout file which also includes the dut logs are very verbose and noisy.

We have manually installed it in the sonic-mgmt docker in our organization and tuned the pytest settings to produce very helpful and concise logs.

pytest-html plugins can be used to post-process the output in various ways based on our different and unique organizational needs.

Hence proposing to add this pkt to the docker file

dockers/docker-sonic-mgmt/Dockerfile.j2

@@ -55,6 +55,7 @@ RUN pip install cffi==1.10.0 \
                 pyro4 \
                 pysnmp==4.2.5 \
                 pytest-repeat \
+                pytest-html \
                 pytest-xdist==1.28.0 \
                 pytest==4.6.5 \
                 redis \