Add check in migration scripts to create new tables only if one doesnt already exist

[anushka-singh]

Alembic upgrades can fail/get stuck/overwrite existing tables if we dont add a check to see the existence of table with same name already.

We can add a check like this:

if not has_table('dataset_bucket', engine):
    op.create_table(
                'dataset_bucket',)

before creating new tables in the migration script upgrade.

Reference: https://github.com/awslabs/aws-dataall/pull/848/files/06edb53d772132ede68201d5d24f9acf731cab98#r1388384179

[dlpzx]

This is a great idea @anushka-singh, thanks for raising an issue. We will include this pattern in any nwe migration script. And if we have bandwidth we will pick it up for existing scripts