osdmaptool: add --test-map-pgs mode

This command will map all pgs from all pools (or just one pool) to osds
and summarize the placement and calculate the actual standard deviation and
the expected value.

Signed-off-by: Sage Weil <sage@inktank.com>
osdmaptool: allow a completely random placement

This useful for comparison purposes and sanity-checking the results.

Signed-off-by: Sage Weil <sage@inktank.com>
osdmaptool: fix cli tests

Signed-off-by: Sage Weil <sage@inktank.com>
osdmaptool: s/simple.t/missing-argument.t/

Signed-off-by: Loic Dachary <loic@dachary.org>
osdmaptool: test --import/export-crush

Signed-off-by: Loic Dachary <loic@dachary.org>
osdmaptool: tests for --test-map-pgs

Signed-off-by: Loic Dachary <loic@dachary.org>