confd: fault injection support

[troglobit]

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: ...
   }