Abort crowbar_join if chef-server is not answering on admin server

There is no point in trying to continue, since the chef-client will
fail.
Skip time synchronization when NTP server is down

This avoids blocking on it forever.
Avoid infinite loops with sntp

We do it 120 times, and stop looping afterwards.

It is quite a long time in the worst case: timeout seems to be 15
seconds, so that'd be ~30 minutes. However, worst case should be avoided
since we don't even enter the loop if the NTP port is not open on the
server.

If the NTP server replies but without time, that means we try during 2
minutes, which is not too bad.
Make crowbar_join work with multiple NTP servers
Make crowbar_register know about multiple NTP servers
Make sure that netcat is installed

We add it to the autoyast profile and to crowbar_register, this way
we're 100% sure that it's available for crowbar_join.Make sure
Merge pull request #291 from vuntz/admin-server-down

Better handle situation when ntp / chef-server don't reply in crowbar_join