Werk #4039: Fix UnicodeEncodeError exception in WATO folder display

This fixes the following exception, which happens in some situations when using the WATO hosts management:

Traceback (most recent call last):
File "/omd/sites/heute/share/check_mk/web/htdocs/wato.py", line 263, in page_handler
modefunc("content")
File "/omd/sites/heute/share/check_mk/web/htdocs/wato.py", line 482, in mode_folder
show_hosts(folder)
File "/omd/sites/heute/share/check_mk/web/htdocs/wato.py", line 805, in show_hosts
table.cell(_("Permissions"), ", ".join(map(render_contact_group, permitted_groups)))
File "/omd/sites/heute/share/check_mk/web/htdocs/table.py", line 112, in cell
finish_previous()
File "/omd/sites/heute/share/check_mk/web/htdocs/table.py", line 83, in finish_previous
next_func(*next_args[0], **next_args[1])
File "/omd/sites/heute/share/check_mk/web/htdocs/table.py", line 119, in add_cell
text = str(text)
UnicodeEncodeError: 'ascii' codec can't encode character u'\xfc' in position 60: ordinal not in range(128)

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