cobbler: Change method used to ping Cobbler host in rc.local

cobbler: Change method used to ping Cobbler host in rc.local

I've observed a *very* occasional race condition where dhclient
completes but the host can't ping Cobbler.  Instead of timing out
waiting for one ping packet to return, we'll try pinging X number of
times (based on $attempts number) and then give up.

I'll paste an example of the race condition observed in the PR notes.

Signed-off-by: David Galloway <dgallowa@redhat.com>