switch list_pkgs on yum-based systems to using the rpmdb instead of a cmd.run
switch list_pkgs on yum-based systems to using the rpmdb instead of a cmd.run

the rest of this module likes to use YumBase(), so do that for list_pkgs
as well.  this is just a straight-forward switch to how the package list is
queried, it doesn't change anything functionality-wise.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt