tests: verify erasure code read / write after remapping

tests: verify erasure code read / write after remapping

Implement the following scenario:

 * create an erasure coded pool and write an object

 * mark two osd used to store the object out
 * wait until the PG is clean
 * read the object and compare it with the original

 * mark the two osd back in
 * wait until the PG is clean
 * read the object and compare it with the original

The scenario is run with the jerasure plugin only because the code path
involved does not vary depending on the erasure code plugin.

http://tracker.ceph.com/issues/11667 Fixes: #11667

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