test to upload rpm content to an existing repo

@Ichimonji10 : could you please review it once. I'm facing below issue, it looks like to me a bug in api's but still wanted to double check:

```
FAIL: @Test: Create a repository and upload rpm contents
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/sghai/QE/robottelo/tests/foreman/api/test_repository_v2.py", line 366, in test_positive_update_5
    status_code_error(path, status_code, response),
AssertionError: Desired HTTP 200 but received HTTP 500 after sending request to https://qe-sat6-rhel7.usersys.redhat.com:443/katello/api/v2/repositories/145/upload_content. [u"undefined method `path' for #<String:0x007f9f3d061330>"]
```

Nice and idiomatic. Thanks for updating the docs, too.