mon/OSDMonitor: test crush with fork but not crushtool spawn

mon/OSDMonitor: test crush with fork but not crushtool spawn

We see timeouts here, but I very much suspect they are due to the overhead
of launching the crushtool process and not the test itself.  We have
perfectly code already in our process, though; we just want to isolate
failure and time out reliably.  So, fork and timeout, without executing
a new binary.

Hopefully-fixes: http://tracker.ceph.com/issues/19964
Signed-off-by: Sage Weil <sage@redhat.com>