Return success when registrying unhealthy node.

Fix #2192
Corrent.  That's part of the larger change I wanna make :)

At initial stage, I plan to have apiserver filter out unhealthy node to keep the interface the same, i.e. scheduler doesn't have to take care of filtering out unhealthy node.  Later, we can use something similar to a selector.  So scheduler can select healthy node, admin can find and diagonse unhealthy node.  We can probabaly add more states later.