Rename IsEtcdConflict to IsEtcdTestFailed

IsEtcdConflict checks for the etcd errror "TestFailed" for the conflict error name.
https://github.com/coreos/etcd/blob/a563d82f95d60c6f3ddf3619ed150e09856daf2f/Documentation/errorcode.md

Bonus:
- Replaces magic numbers with const.
- Rename test function to have format: Test[TargetFuncName]
- Method name should come first in its doc comment.
I don't see this const defined anywhere, did I miss a file? or are they in the etcd client code?