Improve codecov integration

[etj]

• codecov seems to be run twice for each batch of tests:
  

  geonode/.circleci/config.yml

  Line 95 in dc6268c

  docker-compose --env-file .env_test -f docker-compose-test.yml exec django bash -c 'codecov; bash <(curl -s https://codecov.io/bash) -t 2c0e7780-1640-45f0-93a3-e103b057d8c8'

  
  According to https://github.com/codecov/python-standard#usage you either call

  bash <(curl https://codecov.io/bash)

  OR

  pip install codecov
  codecov

• We may use the flags feature to better check the results for the various test suites
• Investigate whether the various test suites really add up to the final coverage results.