Avoid using too generic names in _Namespace

As _Namespace is used to hold key-values for user options,
avoid using conflict-prone names for internal purposes.
Namely, prefix ours with '_'.

For example, consider the following.

    conf.register_cli_opt(cfg.BoolOpt('parser'))

In this case, the option would get added to the namespace
object as a "parser" attribute, which would conflict with
the existing name in the object.

Change-Id: Ibaf8408a118e07a8a598f87ac10a91312683df40
Closes-Bug: #1284969