DOCS-3202 exclude new package names to install pre-2.6 RPMs

As of 2.6.1 and SERVER-13563, we identify old RPM package names as
being obsoleted by new RPMs. This means that in order to install
RPM's using the old names (mongo-10gen, mongo-10gen-server,
mongo-10gen-enterprise, mongo-10gen-enterprise-server) requires
telling yum to ignore the new packages so it doesn't see that they
obsolete the old ones.