app/quantum_adapter: Lazily initialize neutron client

When ryu starts up, related openstack components (keystone and neutron)
might not be running. They might be during start up process.
In that case, quantum_adapter results in exception as follows.
So in order to avoid ordering of starting up, lazily initialize neutoron
api.

> hub: uncaught exception: Traceback (most recent call last):
>   File "/ryu/lib/hub.py", line 48, in _launch
>     func(*args, **kwargs)
>   File "/ryu/base/app_manager.py", line 173, in _event_loop
>     handler(ev)
>   File "/ryu/app/quantum_adapter.py", line 398, in dp_handler
>     ovs_switch = self._get_ovs_switch(dpid)
>   File "/ryu/app/quantum_adapter.py", line 381, in _get_ovs_switch
>     ovs_switch = OVSSwitch(dpid, self.nw, self.ifaces, self.logger)
>   File "/ryu/app/quantum_adapter.py", line 167, in __init__
>     token = _get_auth_token(logger)
>   File "/ryu/app/quantum_adapter.py", line 90, in _get_auth_token
>     httpclient.authenticate()
>   File "/neutronclient/client.py", line 211, in authenticate
>     content_type="application/json")
>   File "/neutronclient/client.py", line 141, in _cs_request
>     raise exceptions.ConnectionFailed(reason=e)
> ConnectionFailed: Connection to neutron failed: [Errno 111] ECONNREFUSED

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>