Fix portage-provider so that packages with keywords can be installed

This patch removes the --stable restriction from the portage provider,
which made the installation of packages with keyword-restrictions impossible.
By superimposing this restriction the provider broke in many installations
and thus rendered puppet unusable for systems that relied on packages with
keyword-restrictions. This applies for example to all packages provided by
overlays.
This fix puts the administrator back into the responsibility to decide which
packages he/she wants to install.