ErasureCodePlugin: plugin registry tests and example

ErasureCodePlugin: plugin registry tests and example

libec_example.la is a fully functional plugin based on
ErasureCodeExample to test the ErasureCodePlugin abstract
interface. It is dynamically loaded to test the
ErasureCodePluginRegistry implementation.

Although the plugin is built in the test directory, it will be
installed. noinst_LTLIBRARIES won't build the shared library, only the
static version which is not suitable for testing.

http://tracker.ceph.com/issues/5878 refs #5878

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