Enable skipped tests from test_models.py

Enable test_modelbase_iteritems and test_modelbase_iter which were
always skipped. These tests need a mapped object instead of
ModelBase instance.

Refactor ModelBaseTest a bit - move creation of ModelBase and
ExtraKeysModel instances to setUp().

Refactor test_modelbase_iteritems and test_modelbase_iter
to check if we can iterate over columns and extra_keys.

Co-Authored-By: Oleksii Chuprykov <ochuprykov@mirantis.com>

Closes-Bug: #1312220

Change-Id: If19137cc81959031764c1b78f990b67a8912a304
Merge "Enable skipped tests from test_models.py"