Fix install hook error

Without this fix the unit_tests/ folder is considered part
of the interface. The reactive boilerplate then tries to
import it and fails on missing dependencies. This solves [1].

[1]: https://github.com/openstack-charmers/magpie-layer/issues/24

Change-Id: If0d2b119694e42095aefe49e57ba49bfc5b089ce

.gitreview

@@ -0,0 +1,4 @@
+[gerrit]
+host=review.opendev.org
+port=29418
+project=openstack/charm-interface-magpie

.zuul.yaml

@@ -0,0 +1,4 @@
+- project:
+    templates:
+      - python35-charm-jobs
+      - openstack-python3-ussuri-jobs

interface.yaml

@@ -2,3 +2,10 @@ name: magpie
 summary: Interface used by the Magpie charm layer for peer network tests
 version: 1
 maintainer: OpenStack Charmers <openstack-discuss@lists.openstack.org>
+ignore:
+  - 'unit_tests'
+  - '.testr.conf'
+  - 'test-requirements.txt'
+  - 'tox.ini'
+  - '.gitignore'
+  - '.gitreview'