ISSUE-9682: CI cpp-tests is broken now for the latest pulsar-build image

[sijie]

Original Issue: apache#9682

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Describe the bug
Currently, the CI for cpp tests is broken because it uses the apache/pulsar-build:ubuntu16.04 image as the base image to build cpp client. However, the image was updated yesterday to contain the change of apache#9609. This PR add python3 and pip3 and then CMake found python3 by default and tried to find boost python for python3. However, the boost python of pulsar-build image was installed by libboost-all-dev, which only supports python2.

To Reproduce
See recent PR's CI.

-- Found the following Boost libraries:
--   system
-- Found PythonLibs: /usr/lib/x86_64-linux-gnu/libpython3.5m.so (found version "3.5.2") 
-- PYTHON: 3.5.2
-- DETECTED Python 3
-- Boost version: 1.58.0

CMake Error at python/CMakeLists.txt:84 (MESSAGE):
  Could not find Boost Python library

Expected behavior
The pulsar-build image should work for cpp-tests.

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