Copy column that's already attached

Copy column that's already attached

Modified the ``add_column()`` operation such that the ``Column`` object in
use is shallow copied to a new instance if that ``Column`` is already
attached to a ``table()`` or ``Table``. This accommodates for the change
made in SQLAlchemy issue #5618 which prohibits a ``Column`` from being
associated with multiple ``table()`` objects. This resumes support for
using a ``Column`` inside of an Alembic operation that already refers to a
parent ``table()`` or ``Table`` as well as allows operation objects just
autogenerated to work.

Change-Id: Idc6933cf0da40373f4f270d75883995822c072f0
Fixes: #753