Use yumbase.rpmdb for list_pkgs()

I all-too-hastily reverted a change @mchesnut made in 585f7cc. The
discussion regarding this can be found here:

https://github.com/saltstack/salt/issues/2884#issuecomment-12012256

This commit puts back @mchesnut's change, with the exception of the
"args" bits. Accepting arbitrary arguments in list_pkgs is not supported
uniformly across pkg providers, and in addition I am working to make
pkg.version do exactly this across all providers, in order to facilitate
multi-pkg support for the pkg.latest state. This makes passing arguments
to pkg.list_pkgs redundant. I have replaced refs to list_pkgs which do
this with refs to version.