Move get_table() from test_migrations to sqlalchemy.utils

get_table() is very useful in different places. So it is better
to move it from test_migrations to sqlalchemy.utils.

blueprint db-common-migration-and-utils

Change-Id: I1191b95aede8688a2b38ac4dd0f2e9108dda1559
Merge "Move get_table() from test_migrations to sqlalchemy.utils"