slapgrid: fix _updateCertificate in python3

Traceback (most recent call last):
  File "/opt/slapos/eggs/slapos.core-1.7.10-py3.7.egg/slapos/grid/slapgrid.py", line 1435, in processComputerPartitionList
    self.processComputerPartition(computer_partition)
  File "/opt/slapos/eggs/slapos.core-1.7.10-py3.7.egg/slapos/grid/slapgrid.py", line 1329, in processComputerPartition
    raise e
  File "/opt/slapos/eggs/slapos.core-1.7.10-py3.7.egg/slapos/grid/slapgrid.py", line 1254, in processComputerPartition
    local_partition._updateCertificate()
  File "/opt/slapos/eggs/slapos.core-1.7.10-py3.7.egg/slapos/grid/SlapObject.py", line 513, in _updateCertificate
    fout.write(new_content)
TypeError: a bytes-like object is required, not 'str'

Also add test for the failure.

slapos/grid/SlapObject.py

@@ -509,7 +509,7 @@ class Partition(object):
         else:
           self.logger.info('Changed %s content. Updating %r' % (name, path))
 
-        with os.fdopen(os.open(path, os.O_CREAT | os.O_WRONLY | os.O_TRUNC, 0o400), 'wb') as fout:
+        with os.fdopen(os.open(path, os.O_CREAT | os.O_WRONLY | os.O_TRUNC, 0o400), 'w') as fout:
           fout.write(new_content)
         os.chown(path, uid, gid)