Re-vamp Roles file

Puppet swallows captures both stdout and stderr of the external node
classifier, making the verbose validation errors emitted by puppet-classifier
tragic and worthless.

This change makes the parsing of the file far more forgiving (it no longer
requires valid YAML input -- any line-oriented list of class names will do) and
it recovers from almost all errors, preferring to emit an empty
roster than to crash.

Change-Id: I2f805dbdead5b6ebde9dce231fef8e16f7f7362a