Update Dockerfile for python3

Dockerfile

@@ -2,7 +2,7 @@ FROM blazemeter/taurus
 
 ADD test.jmx /tmp/test.jmx
 RUN sed -ibkp -e 's/^.*bintray.*$//g' /etc/apt/sources.list ; cat /etc/apt/sources.list
-RUN apt-get update && apt-get -y install python
+RUN apt-get update && apt-get -y install python3
 RUN curl https://jmeter-plugins.org/repo/ | python -c 'import json; import sys; sys.stdout.write(",".join([x["id"] for x in json.loads(sys.stdin.read()) if "id" in x.keys() and not x["id"].startswith("ulp")]))' > plugins.lst && cat plugins.lst
 RUN bzt -o modules.jmeter.path=/jmeter /tmp/test.jmx -o modules.jmeter.plugins=[$(cat plugins.lst)] || true
 RUN ls /jmeter/lib/ext/jmeter-plugins-casutg-*.jar && rm -rf /tmp/*