Fix test_create_host failure

- create_host doesn't return 403 for user_lab but 200 because the form
  validation fails (the network_id is not part of the form choices)
- caching disabled during tests
  the current_user could sometimes return a different username because of
  caching: the database is cleaned between each test, so different users
  can have the same id and an old user could be returned instead of the new
  one due to caching

JIRA INFRA-578