DB engine specs cannot change the CTE_ALIAS variable

[alexclavel-ocient]

As it stands, the CTE_ALIAS used in superset/connectors/sqla/models.py cannot be changed by the db engine spec. Instead, it pulls the value from the base model.

This causes a problem for databases that cannot use the default CTE_ALIAS of '__cte'. In particular, this alias starts with an underscore, which may not be supported

How to reproduce the bug

Create Chart on a dataset containing a CTE

Expected results

Generate query like:
WITH cte as (SELECT * from schema.table),
MY_CTE_ALIAS AS (SELECT * from cte)
SELECT id AS "id"
FROM MY_CTE_ALIAS
LIMIT 1000;

Actual results

WITH cte as (SELECT * from schema.table),
MY_CTE_ALIAS AS (SELECT * from cte)
SELECT id AS "id"
FROM __cte
LIMIT 1000;

Checklist

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.