Rename hbase_classpath -> hbase_classpath_str.

Also extract hbase_classpath, which parses hbase_classpath_str by
splitting on the path delimiter, tossing empty entries, those for
problematic jars (slf4j and patched junit) and normalizing the others
with `File.expand_path`.

The normalization should help make our tests more robust across
platforms (i.e. hbase classpath has different species of oddities on
linux vs osx.)