config_parse.py: use a `_YamlConfigParser` object a single time
config_parse.py: use a `_YamlConfigParser` object a single time

This patch changes `_YamlConfigParser` so that you can only use it once:
the object parses the file on construction, and then makes the
configuration object available through its `config` property.

Because you can't parse another file with the same parser, we don't need
to care about any stale state.

Signed-off-by: Philippe Proulx <2096628897b40c93960fdd9e24c9c883a54d4fe9@gmail.com>

efficios/barectf,efficios/barectf