extend rndc timeout to 60 seconds

the idle timeout for rndc connections was set to 10 seconds, but this caused intermittent system failures of the 'rndc' system test on slow platforms, since 'rndc reconfig' could time out before reconfiguration was complete. this commit restores the original timeout value of 60 seconds, which was changed inadvertently after rndc was updated to use the network manager. the system test has also been cleaned up.

Closes #2643 (closed)