SSSCTL: Add cache-expire command
    
    Add sssctl cache-expire as a wrapper for the sss_cache utility to
    invalidate cached objects.
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Michal Židek <mzidek@redhat.com>