intg: Fix creating of slapd configuration
    
    The python module subprocess expect bytes as an input.
    
    Traceback (most recent call last):
      File "src/tests/intg/test_ldap.py", line 51, in ds_inst
        ds_inst.setup()
      File "src/tests/intg/ds_openldap.py", line 201, in setup
        self._setup_config()
      File "src/tests/intg/ds_openldap.py", line 169, in _setup_config
        slapadd.communicate(config)
      File "/usr/lib64/python3.6/subprocess.py", line 821, in communicate
        self._stdin_write(input)
      File "/usr/lib64/python3.6/subprocess.py", line 776, in _stdin_write
        self.stdin.write(input)
    TypeError: a bytes-like object is required, not 'str'
    
    Reviewed-by: Martin Basti <mbasti@redhat.com>
    (cherry picked from commit 73c9e3d710434857b5e13d5ccfc1f1245b1d2997)