installutils: improve directive value parsing in `get_directive`
    
    `get_directive` value parsing was improved in order to bring its logic
    more in-line to changes in `set_directive`: a specified quoting
    character is now unquoted and stripped from the retrieved value. The
    function will now also error out when malformed directive is
    encountered.
    
    https://fedorahosted.org/freeipa/ticket/6991
    
    Reviewed-By: Tomas Krizek <tkrizek@redhat.com>