Add zanata cache to git ignore

For translation job, we need to ignore produced files and
folders, otherwise
git-review will fail due to uncommited changes.

Change-Id: Id79539ec6cb93a6ae635aed43ef0a5e6f0cede34
Signed-off-by: Dmitriy Rabotyagov <noonedeadpunk@gmail.com>

.gitignore

@@ -32,3 +32,8 @@ node_modules
 
 # Coverage data
 .coverage.*
+
+# ignore zanata/sphinx cache on translation job
+.zanata-cache/
+doc/source/.doctrees/
+upper-constraints.txt