ERT-769: Moved load_segment flag further in.

This change was introduced to ensure that the is_MSW flag is correctly
set on a per-well basis, even when the load_segment flag is false. The
commit also incldues reverting some test asserts in the Python test.
Merge pull request #633 from joakim-hove/769-fixup

ERT-769: Moved load_segment flag further in.