chore(python): modify templated noxfile to support non-cloud APIs

[parthea]

Currently the owlbot.py file for non-cloud APIs need to have replacements in owlbot.py in order to use the templated noxfile. See the example from python-analytics-admin here which has:

s.replace("noxfile.py",
'''["']--cov=google/cloud["'],''',
'''"--cov=google.analytics",''')

This PR eliminates the need for this replacement in owlbot.py. I tested this change in googleapis/python-orchestration-airflow#41, and googleapis/python-analytics-admin#157 and the Kokoro build passed in both.