erasure-code: add directory to plugin init functions

erasure-code: add directory to plugin init functions

The prototype of the init functions of erasure coded plugins is changed
from

   int __erasure_code_init(char *plugin_name)

to

   int __erasure_code_init(char *plugin_name, char *directory)

The jerasure plugin will find optimized variants in this directory and
load them. The load() and preload() functions of
ErasureCodePluginRegistry only use a directory instead of a more generic
parameters map. The parameters map was only used for the directory entry
anyway.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>