Redefine TargetTagsDict.get() to always use __missing__

The code uses `target_tags.get(branch, 'rawhide')` and dict.get()
doesn't use __missing__, only dict[key] access does.

Patch from Miro Hrončok provided at:
https://pagure.io/fedora-ci/simple-koji-ci/pull-request/37

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

roles/simple-koji-ci/templates/simple_koji_ci.py

@@ -3,6 +3,10 @@ class TargetTagsDict(dict):
     def __missing__(self, key):
         return key
 
+    def get(self, key, default):
+        """Normally, get() doesn't go trough __missing__"""
+        return self[key]
+
 
 config = {
     'simple-koji-ci.enabled': True,