_yaml.py: Prevent ruamel from removing underscores

The ruamel parser interprets values such as 1_2_3 as numbers, ignoring
the underscores. However, for `track` we need the value as a verbatim
string. This change configures ruamel to parse all values as strings,
leaving potential conversions to node_get() with the `expected_type`
parameter.

This fixes #166 and adds tests