Make configurable delete request for Bootstrap Server

[sbernard31]

Before this PR, bootstrap server always starts with delete request on "/".
Now in BootstrapConfig, there is a toDelete list of path to object to delete.

[sbernard31]

bs server demo UI create a bootstrap config which will delete "/0" and "/1".
For custom config you can use REST API.

Here a python script as example, to delete "/" as before this PR:

import requests
import json

BASE_URL = "https://leshan.eclipseprojects.io/bs/"  # the URL of the server.
url = BASE_URL + "api/bootstrap/clientendpoint"  # the client endpoint here !
data = {
        "toDelete": ["/"],
        "servers": {"0": {"shortId": 123,
                        "lifetime": 20,
                        "defaultMinPeriod": 1,
                        "defaultMaxPeriod": None,
                        "disableTimeout": None,
                        "notifIfDisabled": True,
                        "binding": "U"}},
        "security": {"0":{"uri": "coaps://leshan.eclipseprojects.io:5784",
                        "bootstrapServer": True,
                        "securityMode": "PSK",
                        "publicKeyOrId": [98,111,111,116,115,116,114,97,112,95,99,108,105,101,110,116,95,105,100,101,110,116,105,116,121], #binary represenation of "bootstrap_client_identity"
                        "serverPublicKey" : [],
                        "secretKey": [115,101,99,114,101,116,95,107,101,121], #binary representation of "private_key", the hexa value is 7365637265745f6b6579
                        "smsSecurityMode": "NO_SEC",
                        "smsBindingKeyParam" : [],
                        "smsBindingKeySecret" : [],
                        "serverSmsNumber" : "+3343577911",
                        "serverId" : 0,
                        "clientOldOffTime" : 20},
                   "1":{"uri": "coaps://leshan.eclipseprojects.io:5684",
                        "bootstrapServer": False,
                        "securityMode": "PSK",
                        "publicKeyOrId": [115,101,99,117,114,101,95,99,108,105,101,110,116,95,105,100],#represenation of "secure_client_id"
                        "serverPublicKey" : [],
                        "secretKey": [112,114,105,118,97,116,101,95,107,101,121], #binary representation of "private_key", the hexa value is 707269766174655f6b6579
                        "smsSecurityMode": "NO_SEC",
                        "smsBindingKeyParam" : [],
                        "smsBindingKeySecret" : [],
                        "serverSmsNumber" : "+3343577464",
                        "serverId" : 123,
                        "clientOldOffTime" : 1}},
        "acls": {"0":{
                        "objectId": 3,
                        "objectInstanceId": 0,
                        "acls": {3333: 1},
                        "AccessControlOwner":123}}
        };

headers = {'Content-type': 'application/json', 'Accept': 'text/plain'}
r = requests.post(url, data=json.dumps(data), headers=headers)
print r.status_code
print r.content