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.
Make `AuthSourceLDAP.read` work

Expand the signature of method `EntityReadMixin.read` with a new parameter:
`ignore`. This new argument allows a user to specify that certain attributes
should not be read from the server. Update all overrides of the `read` method to
accomodate this change.

Define `AuthSourceLDAP.read`. This allows the method to succeed, and it fixes a
failing Jenkins test.

    $ nosetests tests/foreman/api/test_multiple_paths.py -m test_entity_read_1_AuthSourceLDAP
    .
    ----------------------------------------------------------------------
    Ran 1 test in 1.534s

    OK