osd crush rule create-erasure

proposal for "osd crush rule create-erasure" which provides the same service as "osd crush rule create-simple" by delegating the creation to the erasure code plugin. Although a sensible default could be hardcoded for the jerasure plugin, it is significantly more complex for the pyramid plugin.

The erasure code plugin loader is isolated in a convenience library so that it can be loaded from the mon to help compute the ruleset without requiring a link to the OSD library.