minor bugs in lease4 commands

1. lease4-get error states "duid" and not "client-id"

{"command": "lease4-get",
           "arguments": {"identifier": "ff:01:02:03:ff:05",
                         "identifier-type": "something",
                         "subnet-id": 1}}

Should return: "Incorrect identifier type: something, the only supported values are: address, hw-address, client-id"

Returns: "Incorrect identifier type: something, the only supported values are: address, hw-address, duid"

2. lease4-get-by-client-id error states "duid" and not "client-id"

{"command": "lease4-get-by-client-id",
           "arguments": {"client-id": ""}}

Should return: "Empty client-id is not allowed"

Returns: "Empty DUIDs are not allowed"

3. lease4-get-all accepts negative numbers as subnet number Not necessary a problem.

{"command": "lease4-get-all",
           "arguments": {"subnets": [-1]}}

4. lease6-get accepts "subnet-id" and "identifier" and "identifier-type": "duid" without "iaid" but returns 0 leases found without complaining about lack of iaid example:

{"command": "lease4-get",
           "arguments": {"subnet-id": 1, "identifier": "1a:1b:1c:1d:1e:1f:20:21:22:23:24", "identifier-type": "duid"}}

will always return 0 leases found.