Added retry to sonic-mgmt docker container (#7997)

Why I did it
the motivation for this PR is to add retry_call to several test cases in the community, for example, the following cases:

test_show_platform_fanstatus_mocked
test_show_platform_temperature_mocked
are executing a command once and comparing the output to the expected mock data,
sometimes differences between the mock and the actual are causing the tests to fail.

retry will make these tests more stable.
retry will also be more efficient than sleep which will cause the tests to run longer because sometimes it is not necessary to sleep all that time, retry will only run a function only until it passed.

How I did it
added retry to the docker file

How to verify it
I run the tests with retry on the docker after installing the retry package

Signed-off-by: Sharon Lutati <slutati@nvidia.com>

dockers/docker-sonic-mgmt/Dockerfile.j2

@@ -70,6 +70,7 @@ RUN pip install cffi==1.10.0 \
                 tabulate \
                 textfsm \
                 virtualenv \
+                retry \
     && git clone https://github.com/p4lang/scapy-vxlan.git \
     && cd scapy-vxlan \
     && python setup.py install \