Do not use asserts with business logic

a piece of code in node object was relying on result of 'assert'
to block negative values of properties.

When running Python in optimized mode (-OO) this assert will be
swallowed and negative values may slip through.

Replace this line with proper if-block.

Change-Id: Iec1edc55219cfd6c54afa52c1ecd0cf76340bddf

ironic/objects/node.py

@@ -148,8 +148,9 @@ class Node(base.IronicObject, object_base.VersionedObjectDictCompat):
                 continue
             try:
                 int_value = int(value)
-                assert int_value >= 0
-            except (ValueError, AssertionError):
+                if int_value < 0:
+                    raise ValueError("Value must be non-negative")
+            except (ValueError, TypeError):
                 msg = (('%(param)s=%(value)s') %
                        {'param': param, 'value': value})
                 invalid_msgs_list.append(msg)