'Initialize placement object
servers: list of servers
nodes: list of all nodes
hosts: list of hosts
switches: list of switches
controllers: list of controllers
links: list of links
(all arguments are optional)
returns: server'

def __init__(self, servers=None, nodes=None, hosts=None, switches=None, controllers=None, links=None):

    self.servers = (servers or [])
    self.nodes = (nodes or [])
    self.hosts = (hosts or [])
    self.switches = (switches or [])
    self.controllers = (controllers or [])
    self.links = (links or [])
