Parsing prefix delegation pool from Kea expects invalid format of the pool

The current code assumes that the prefix delegation pool is specified like this:

"pd-pools": [
    {
        "prefix": 2001:db8:8::/56",
        "delegated-len": 96
    }
]

whereas the actual format is:

"pd-pools": [
    {
        "prefix": 2001:db8:8::",
        "prefix-len": 56,
        "delegated-len": 96
    }
]

This parsing must be corrected.