tests: add --{show_,}path to ceph_erasure_code_non_regression

tests: add --{show_,}path to ceph_erasure_code_non_regression

The directory in which the payload is stored is created from the plugin
parameters. The --show-path shows the directory and exits. This
directory can then be used with --path to override the path created from
the plugin parameters. This is useful to verifying that the jerasure
variant optimized with AVX, SSE4 etc. instructions can be used on the
same payload and show no difference at all. In this case the directory
used by the default jerasure variant is used for each variant and the
parameter that sets the variant to use ( --parameters jerasure-variant )
must not be taken into account to figure out the location of the
payload.

http://tracker.ceph.com/issues/9720 Refs: #9720

Signed-off-by: Loic Dachary <ldachary@redhat.com>