Create new Products for every Docker repo.

I noticed that when I ran both tests that created a custom repository
with Docker content, only the first test passed. Turns out that the way
the test suite for Docker tests (API) was created attempted to re-use a
single custom product (PROD1) and then added a custom repository named
'wordpress'. As repository names are unique within a custom product, the
second time a 'wordpress' custom repository was created the test would
fail.

I have updated the existing API tests so that a new custom product is
created for every test that creates Docker-based repositories.

Both tests pass.
Merge pull request #1594 from omaciel/fix-api-docker

Create new Products for every Docker repo.