Add test suite for Fission development workflow to e2e tests

[erwinvaneyk]

Expanding upon #239: the current e2e test suite only uses the minimal functions to deploy and execute Fission functions; many features are not explicitly covered by the tests

To ensure that we do not break the dev workflow (either due to changes in Fission or Workflows) we need to at least cover the following:

• fission function update:
  • src/deploy/code (Fix Fission fn update support #241)
  • entrypoint
  • buildcmd
• fission function delete (though not explicitly tested)
• fission function create is not tested for all parameters:
  • configmap
  • secret
  • entrypoint
  • buildcmd
• fission function logs
• fission function test (not explicitly tested)
  • all methods
  • body (fission/fission-workflows/test/e2e/tests/test_fission_python.sh)
  • headers (fission/fission-workflows/test/e2e/tests/test_fission_python.sh)
  • body (fission/fission-workflows/test/e2e/tests/test_fission_python.sh)
  • query (fission/fission-workflows/test/e2e/tests/test_fission_python.sh)
• fission function get
• fission spec apply
• fission spec destroy