Allow injecting replication errors
Allow injecting replication errors

This can be controlled by 2 variables:
* `BACKBEAT_INJECT_REPLICATION_ERROR_RATE` for the occurence rate;
* `BACKBEAT_INJECT_REPLICATION_ERRORS` to select the operations which
may be affected.

Issue: BB-280

scality/backbeat,scality/backbeat,scality/backbeat,scality/backbeat