Use implicit nullable=True in sqlalchemy model

In sqlalchemy nullable defaults to True, so we don't need to explicitly
set the value to the default.  By removing all nullable=True everywhere
it makes it easier to spot the nullable=False and also should lead to
less confusion when adding a new column (do I need to add nullable or
not, only some other columns do it)

Change-Id: I22fe2d0caca77005553a6a6f0c7cc2d3e8ea5512
Merge "Use implicit nullable=True in sqlalchemy model"