node health check script does not refresh the "reason for blacklisting"

After MAPREDUCE-211, the node health check script does not refresh the "reason for blacklisting".
The steps to reproduce the issue are:

• Blacklist a TT with an error message 'x'
• Change the health check script to return an error message 'y'
• The "reason for blacklisting" still shows 'x'

The impact of this issue is that the feature fails to trap transient errors.