Generate the DDLs with if not exists or if exists

[shaounak]

Modern data warehouses supports create table if not exists or drop table if exists. Is there a way in alembic to run the migrations using this feature. e.g. in the op.create_table, can we create flag. If the value is set for the flag, then issue CREATE TABLE IF NOT EXISTS else issue the default CREATE TABLE statement.

[CaselIT]

hi,

moving to issue to better track it

[CaselIT]

see #1434