[testing] Make python3 the default python in java-test-image (#12130)

- python3 should be default for the image to work
- this has been broken since the base image was changed to ubuntu:20.04

tests/docker-images/java-test-image/Dockerfile

@@ -43,16 +43,13 @@ RUN echo networkaddress.cache.ttl=1 >> /usr/lib/jvm/java-11-openjdk-amd64/conf/s
 
 # /pulsar/bin/watch-znode.py requires python3-kazoo
 # /pulsar/bin/pulsar-managed-ledger-admin requires python3-protobuf
-RUN apt-get install -y python3-kazoo python3-protobuf
-# use python3 for watch-znode.py and pulsar-managed-ledger-admin
-RUN sed -i '1 s/.*/#!\/usr\/bin\/python3/' /pulsar/bin/watch-znode.py /pulsar/bin/pulsar-managed-ledger-admin
-
-# required by gen-yml-from-env.py
-RUN apt-get install -y python-yaml
+# gen-yml-from-env.py requires python3-yaml
+# make python3 the default
+RUN apt-get install -y python3-kazoo python3-protobuf python3-yaml \
+    && update-alternatives --install /usr/bin/python python /usr/bin/python3 10
 
 RUN apt-get install -y supervisor procps curl less netcat dnsutils iputils-ping
 
-
 RUN mkdir -p /var/log/pulsar \
     && mkdir -p /var/run/supervisor/ \
     && mkdir -p /pulsar/ssl