add alternative package method as work around for yum_rpm named package ...

...problem

When using yum_rpm with something like "pidgin" and the version and architecture are not supplied, the user still only wants to install pidgin, in the same way as doing yum install pidgin. Since we match using name-version-architecture in the yum_rpm package method, pidgin expands to pidgin-.* in the regex match. This causes all packages that have pidgin- as its prefixed to be installed. This proposed workaround package method restores an option that was available in version 2 of CFEngine without affecting existing standard library functionality.