py3: MultiType.validate() now catchs TypeError
py3: MultiType.validate() now catchs TypeError

On Python 3, calling wtypes.validate_value() can fail with TypeError.
Example of error:

  File "solum/api/controllers/v1/datamodel/types.py", line 109, in validate
    return wtypes.validate_value(t, value)
  File ".../site-packages/wsme/types.py", line 370, in validate_value
    return datatype.validate(value)
  File ".../site-packages/wsme/types.py", line 166, in validate
    if self.minimum is not None and value < self.minimum:
  TypeError: unorderable types: list() < int()

Partially implements: blueprint solum-python3
Change-Id: I5eb0ac74d4c0df47a87d2fb8374b680b89ebd45c

devdattakulkarni/test-solum,devdattakulkarni/test-solum,openstack/solum,stackforge/solum,openstack/solum,stackforge/solum