Make the datetime utility function coerce to UTC

Getting datetime objects from existing sources (i.e. sqlalchemy)
is problematic because they're "timezone naive". Make the utility
function coerce these to UTC for easier conversion.

Related to blueprint unified-object-model

Change-Id: I61bc6b13f9bd9441d64cc6930e43e921af72a342
Merge "Make the datetime utility function coerce to UTC"