Need to quote some keys for yang.

The Kea6 reservations.json example file use 'somevalue' as the identifier of a (flex-id) host reservation. Yang uses the same character ' for list keys so it conflicts. The solution should be to convert the identifier in hexadecimal so:

• check the textual and hexadecimal forms can be used together / safely.
• check presence of problematic characters in a string used as a list key
• create an adaptor to quote or convert strings used as list key. Nothing hard but low priority as this is clearly a corner case.