crowbar_join: Try longer to ping the admin server

Apparently, on some hardware, the network card takes 20 seconds to be
up, so just one initial ping is not a good enough test. Let's try for
two minutes.

https://bugzilla.novell.com/show_bug.cgi?id=871460
crowbar_join: Get the hostname before pinging the admin server
crowbar_join: Do not sleep if we're going to fail anyway
Merge pull request #293 from vuntz/admin-server-down

crowbar_join: Try longer to ping the admin server