confd: fault injection support

For purposes of testing it would be beneficial to support fuzzing the API to sysrepo.  E.g., setting a failure rate for an XPath in the operational datastore.

Possible API:

 - `/system/test/fault-injection <F>`
 - Where `<F>` is a list of faults:

        {
              xpath: ...
              event: .....
              rc: ...
        }

