Cleanup Cross-Validation: Print the xval model keys in JSON and HTML.
Only show cross-val model keys after they are built, otherwise show "In progress".
Fix NPE.
After checkpoint restart, remove previous cross-validation model keys (they are no longer useful).
Merge branch 'master' of https://github.com/0xdata/h2o
Make dataKey/testKey to be the source._key and validation._key, such that the
right frames are referenced from the model page.