as there is very low delay in LAN, I added a timeout to make the detection more quickly
Merge branch 'master' of https://github.com/Damon-wenc/Web_based_test_system
two adjustments about interval
two modifications:
1. while connecting to a ssh server for the first time, a 'confirm action' is needed;
2. I made some adjustments about timeout mechanism, because if the devices are under great pressure, the response of the ssh servers may also get slow.