base_installer: Allow setting cleanup parameter per variant

We're having trouble to make both RHEL 6 and Fedora 18
koji/brew based installs work with the same cleanup
settings. So, allow to pass a cartesian param
installer_cleanup = yes/no to override the default
parameters being passed to the install function.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>