intg: Convert output of executed commands to strings
    
    We know that all our tested utilities (sssctl, sss_cache, sss_override)
    returns strings and not binary data. We can safely encode output to
    string (unicode) in python3
    
    Reviewed-by: Martin Basti <mbasti@redhat.com>
    (cherry picked from commit 69f6b919be962b8be78320ffab8607b8d9a0e4c6)