crush: fix crush_calc_straw() scalers when there are duplicate weights

The straw bucket was originally tested with uniform weights and with a
few more complicated patterns, like a stair step (1,2,3,4,5,6,7,8,9).  And
it worked!

However, it does not behave with a pattern like
 1, 2, 2, 3, 3, 4, 4

Strangely, it does behave with
 1, 1, 2, 2, 3, 3, 4, 4

and more usefully it does behave with
 1, 2, 2.001, 3, 3.001, 4, 4.001

That is, the logic that explicitly copes with weights that are duplicates
is broken.

The fix is to simply remove the special handling for duplicate weights --
it isn't necessary and doesn't work correctly anyway.

Add a test that compares the mapping result of  [1, 2, 2, 3, 3, ...] with
[1, 2, 2.001, 3, 3.001, ...] and verifies that the difference is small.
With the fix, we get .00012, whereas the original implementation gets
.015.

Note that this changes the straw bucket scalar *precalculated* values that
are encoded with the map.  The next time the bucket is modified or
recompiled we will start mapping to new values, but the mapping result of
any given (encoded, stored) map will not change.

Backport: giant, firefly
Signed-off-by: Sage Weil <sage@redhat.com>
mon: add 'osd crush {get,set}-tunable <name> [value]' commands

For now, just add the straw_calc_version tunable.

Signed-off-by: Sage Weil <sage@redhat.com>
wip straw2
crush: move default bucket choice into CrushWrapper helper

Signed-off-by: Sage Weil <sage@redhat.com>
wip