Support yum and rpm in motdgen

[rfairley]

This adds printing information on the system (current version, whether updates are available) from yum and rpm.

This information should be printed only if its descendent package manager is not available from the command line. That is, information from yum only gets printed if dnf is not available, and information from rpm is printed only if yum and dnf are not available as commands.

Adding this would allow motdgen to display information for CentOS7, and any earlier versions of Fedora and CentOS that have only rpm or yum available.

[rfairley]

Irrelevant - functionality removed.