automember-rebuild uses asynchronous 389 task, and returned
success even if the task didn't run. this patch fixes this
issue adding a --nowait parameter to 'ipa automember-rebuild',
defaulting to False, thus when the script runs without it,
it waits for the 'nstaskexitcode' attribute, which means
the task has finished. Old usage can be enabled using --nowait,
and returns the DN of the task for further polling.
New tests added also.
https://fedorahosted.org/freeipa/ticket/4239
Reviewed-By: Petr Viktorin <pviktori@redhat.com>