[SPARK-9956] [ML] Make trees work with one-category features

This modifies DecisionTreeMetadata construction to treat 1-category features as continuous, so that trees do not fail with such features.  It is important for the pipelines API, where VectorIndexer can automatically categorize certain features as categorical.

As stated in the JIRA, this is a temp fix which we can improve upon later by automatically filtering out those features. That will take longer, though, since it will require careful indexing.

Targeted for 1.5 and master

CC: manishamde  mengxr yanboliang

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #8187 from jkbradley/tree-1cat.