Implement UnitTests in tacker-horizon

Tacker has an extension for horizon `tacker-horizon` which provides very limited features, such as showing a list of registered VIMs, VNFDs or so. We don't use this feature so much without checking instances of Tacker quickly via intuitive Web GUI way. So, we have not maintained tacker-horizon so actively, it's enough to use tacker via CUI for many cases for tacker developers. As a result, there are some bugs remained in tacker-horizon for a long time.

One of the reasons why bugs in tacker-horizon have not fixed is that we have no unittests in. Although we can find a bug coincidentally while using the features, but should implement unittests because bugs in horizon are some contextual and not so easy to find by hand.