Patch Pool.DEFAULT_POOL_NAME in BaseOperator

[vshshjn7]

Currently, it is not possible to patch Pool in BaseOperator as the signature sets the default value of Pool as Pool.DEFAULT_POOL_NAME.
Because of this, test case which uses SubDagOperator breaks because of the _validate_pool in subdag_operator.py (L96). Here it tries to query DB to resolve pool conflict, and fails with the following error:

                         def do_execute(self, cursor, statement, parameters, context=None):
                     >       cursor.execute(statement, parameters)
                     E       sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) no such table: slot_pool
                     E       [SQL: SELECT slot_pool.id AS slot_pool_id, slot_pool.pool AS slot_pool_pool, slot_pool.slots AS slot_pool_slots, slot_pool.description AS slot_pool_description 
                     E       FROM slot_pool 
                     E       WHERE slot_pool.slots = ? AND slot_pool.pool = ?
                     E        LIMIT ? OFFSET ?]
                     E       [parameters: (1, 'default_pool', 1, 0)]
                     E       (Background on this error at: http://sqlalche.me/e/e3q8)
                     
                     .pants.d/requirements/requirements/CPython-3.6.4/4f9ce35f4559abaae23151dad89c7103961dd1eb/.deps/SQLAlchemy-1.3.8-cp36-cp36m-macosx_10_13_x86_64.whl/sqlalchemy/engine/default.py:552: OperationalError

My assumption here is that DB doesn't exist for running test cases, hence I am patching Pool.DEFAULT_POOL_NAME to None, so that it won't enter the _validate_pool of subdag_operator.

Here is a gist link of what I am trying to do:
https://gist.github.com/vshshjn7/cdf869b9c9ef5a73c4ffbf73b498c71d

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[boring-cyborg]

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[msumit]

@vshshjn7 one test case is failing due to change. Also, you need to resolve the conflict.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!