erasure-code: the plugin is in a convenience library

So that it can be used by mon without linking with libosd

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: osd crush rule create-erasure

delegates the creation of the rule to the erasure code plugin

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: add crush ruleset create API

because only the plugin knows enough to create a sensible default
ruleset. It is not made pure virtual yet because the jerasure
implementation is not done.

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: example implementation of create_ruleset

by the example plugin used for testing

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