ErasureCodeJerasure: plugin

Create the class matching the string found in the
erasure-code-technique parameter, using the same strings are the
original {encoder,decoder}.c examples from Jerasure-1.2A. Registers
the plugin in ErasureCodePluginRegistry.

https://github.com/dachary/ceph/tree/wip-5879 refs #5879

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