Allow disabling periodic committing when inserting rows with DbApiHook

```
>>> i = 1
>>> commit_every = 0
>>> bool(i % commit_every == 0)  # previously
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
>>> bool(commit_every and i % commit_every == 0)  # with this change
False
```

airflow/hooks/dbapi_hook.py

@@ -156,7 +156,7 @@ def insert_rows(self, table, rows, target_fields=None, commit_every=1000):
                 target_fields,
                 ",".join(values))
             cur.execute(sql)
-            if i % commit_every == 0:
+            if commit_every and i % commit_every == 0:
                 conn.commit()
                 logging.info(
                     "Loaded {i} into {table} rows so far".format(**locals()))