Fixed UnicodeEncodeError for non-ASCII symbols in servel model

Fixing the following error for the server model with UTF-8 symbols, like "DELL PowerEdge™ R720":

```Traceback (most recent call last):
  File "./hetznerctl", line 261, in <module>
    Main()
  File "./hetznerctl", line 60, in __init__
    cmd(argv, self)
  File "./hetznerctl", line 52, in __init__
    self.execute()
  File "./hetznerctl", line 156, in execute
    'model': server.product.encode("latin1")
UnicodeEncodeError: 'latin-1' codec can't encode character u'\u2122' in position 14: ordinal not in range(256)```