erasure-code: implement example create_ruleset

And the associated unit tests.

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: implement ErasureCodeJerasure::create_ruleset

It is based on CrushWrapper::add_simple_ruleset, using a "default" root
and "host" failure domain by default. They can be overridden with
erasure-code parameters ( erasure-code-ruleset-root and
erasure-code-ruleset-failure-domain respectively ).

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: test ErasureCodeJerasure::create_ruleset

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