Use SPARK_INDEX_NAME_FORMAT in combine_frames to avoid ambiguity.

[ueshin]

Use SPARK_INDEX_NAME_FORMAT in utils.combine_frames to avoid ambiguity.

>>> ks.options.compute.ops_on_diff_frames = True
>>> kdf = ks.DataFrame({"a": [1, 2, 3], "Koalas": [0, 1, 2]}).set_index("Koalas", drop=False)
>>> kdf.index.name = None
>>> kdf["NEW"] = ks.Series([100, 200, 300])
>>> kdf
Traceback (most recent call last):
...
pyspark.sql.utils.AnalysisException: Reference 'Koalas' is ambiguous, could be: Koalas, Koalas.;

Related to #1647 as well.

[itholic]

LGTM