erasure-code: control jerasure plugin variant selection

erasure-code: control jerasure plugin variant selection

The jerasure-variant parameter is interpreted as the name of the plugin
variant to be loaded regardless of the available CPU features. The
values can be sse3, sse4, generic. It is undocumented and meant for
benchmarking purposes, primarily to force the generic plugin to be
loaded when the sse4 would be chosen.

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