Fix the pep8 E402 (Module level import not at top of file) errors.
For the Alembic files move the definitions of 'revision' and
'down_revision' below the imports. This is now done in Alembic [1]
For other files fix as needed by moving the imports or lines before
the imports.
In a few cases add a "# noqa E402" to whitelist the issue.
[1] dc2aae0374
Change-Id: I48c96d5da0cb747b3ca3fceea9b4ffa85a9ebe22
6b91ba21df