We don't want to leave the cert/key laying around in a tmp dir.  This
was missed when host.cleanup() was moved out of host.rpm() a long time
ago.

For systems we don't support with proper OS packages, a tarball is
better than nothing.  We already generate one, this just allows us to
save it rather than remove it as part of the cleanup process.

RPM remains the default package type.

When we add new package types, using rpmdir will make much less sense.

Running cleanup() from rpm() made it impossible to create a tarball or
other package after rpm() was called because it deleted the tmpdir,
which was made a class variable in ae26808.

This will make it easier to create a tarball distribution using
distutils.

This allows the code to be run directly or used as a module.