Enable system-site-packages for ptf docker and install thrift for test_qos_sai (#12094)

Why I did it
test_sai_qos failed because of the following error:

"stderr_lines": [
        "Traceback (most recent call last):", 
        "  File \"/usr/bin/ptf\", line 522, in <module>", 
        "    test_modules = load_test_modules(config)", 
        "  File \"/usr/bin/ptf\", line 413, in load_test_modules", 
        "    mod = imp.load_module(modname, *imp.find_module(modname, [root]))", 
        "  File \"saitests/switch.py\", line 19, in <module>", 
        "    import switch_sai_thrift", 
        "ImportError: No module named switch_sai_thrift"
    ], 

It's because test_sai_qos runs ptf script which imports switch_sai_thrift, switch_sai_thrift is installed from python-saithrift_0.9.4_amd64.deb.
For master image, the deb file is for python3, but ptf only has virtual python3 environment, that's why we add --system-site-packages to allow virtual env to access system site-packeges.

Add thrift package in docker ptf virtual python3 env, because currently env-python3 doesn't have thrift module which is needed in switch_sai_thrift.

How I did it
Enable --system-site-packages for virtual py3 env in ptf docker and install thrift for test_qos_sai

How to verify it
load and login ptf conatiner
dpkg - i python-saithrift_0.9.4_amd64.deb
source /root/env-python3/bin/activate
python
import switch_sai_thrift.switch_sai_rpc
Signed-off-by: Zhaohui Sun <[email protected]>

Author: ZhaohuiS

dockers/docker-ptf/Dockerfile.j2

@@ -136,7 +136,7 @@ RUN rm -rf /debs \
     && cd /opt             \
     && wget https://raw.githubusercontent.com/p4lang/ptf/master/ptf_nn/ptf_nn_agent.py
 
-RUN python3 -m venv env-python3
+RUN python3 -m venv --system-site-packages env-python3
 
 # Activating a virtualenv. The virtualenv automatically works for RUN, ENV and CMD.
 ENV VIRTUAL_ENV=/root/env-python3
@@ -173,7 +173,8 @@ RUN python3 -m pip install setuptools \
     && pip3 install itsdangerous \
     && pip3 install retrying \
     && pip3 install jinja2 \
-    && pip3 install scapy==2.4.5
+    && pip3 install scapy==2.4.5 \
+    && pip3 install thrift
 
 {% if docker_ptf_whls.strip() -%}
 # Copy locally-built Python wheel dependencies