Render a single "pass" only for UpgradeOps, DowngradeOps

Render a single "pass" only for UpgradeOps, DowngradeOps

Improved the Python rendering of a series of migration operations such that
a single "pass" is rendered for a :class:`.UpgradeOps` or
:class:`.DowngradeOps` based on if no lines of Python code actually
rendered under the operation, rather than whether or not sub-directives
exist. Removed extra "pass" lines that would generate from the
:class:`.ModifyTableOps` directive so that these aren't duplicated under
operation rewriting scenarios.

Change-Id: Ia2547811565ac24ee0f56ce470f96ec567912de3
Fixes: #550