Fix tox-checkbuild CI job

[Changes in zuul configurations]

• playbooks/checkbuild/pre.yaml calls ensure-nodejs role, but the default nodejs version defined in ensure-nodejs role is still 6. nodejs6 repo is no longer available. This is the cause of the job failure. Let’s specify nodejs_version used in horizon by default.
• Ensure to install tox globally in the system. tox -e checkbuild calls “tox” as the test commands, so it needs to be installed globally.
• Specify “python_version” so that zuul ensures to install a required version of python interpreter.

[Changes in cookiecutter template]

• hacking version in test-requirements.txt was too old. It is updated to use more recent version which horizon uses.
• Update python version to 3.9 in tox.ini.
• Install horizon and its dependencies separately. The constraints file contains horizon, so installing horizon along with the constraints file results in a version conflict error.

Change-Id: If4bae3fde1f29eeb1d4d43942acc21ff2ef19482