quantum_adapter: check if quantum_controller_addr is valid

With this, more understandable error will be produced.
Otherwise it results in mysterious error like

transaction error: {"details":"Table Bridge column controller row 69da7121-51fa-4d6e-ad24-18ff9c857b87 references nonexistent row 17c4aa64-ed91-4d8a-a905-a137e6969ad9 in table Controller.","error":"referential integrity violation"}
hub: uncaught exception: Traceback (most recent call last):
  File "/ryu/ryu/lib/hub.py", line 50, in _launch
    func(*args, **kwargs)
  File "/ryu/ryu/base/app_manager.py", line 104, in _event_loop
    handler(ev)
  File "/ryu/ryu/app/quantum_adapter.py", line 383, in conf_switch_set_handler
    self._conf_switch_set_ovsdb_addr(ev.dpid, ev.value)
  File "/ryu/ryu/app/quantum_adapter.py", line 372, in _conf_switch_set_ovsdb_addr
    ovs_switch.set_ovsdb_addr(dpid, value)
  File "/ryu/ryu/app/quantum_adapter.py", line 169, in set_ovsdb_addr
    ovs_bridge.set_controller([self.ctrl_addr])
  File "/ryu/ryu/lib/ovs/bridge.py", line 128, in set_controller
    self.run_command([command])
  File "/ryu/ryu/lib/ovs/bridge.py", line 101, in run_command
    self.vsctl.run_command(commands, self.timeout, self.exception)
  File "/ryu/ryu/lib/ovs/vsctl.py", line 1006, in run_command
    self._run_command(commands)
  File "/ryu/ryu/lib/ovs/vsctl.py", line 999, in _run_command
    self._do_main(commands)
  File "/ryu/ryu/lib/ovs/vsctl.py", line 927, in _do_main
    if self._do_vsctl(idl_, commands):
  File "/ryu/ryu/lib/ovs/vsctl.py", line 898, in _do_vsctl
    vsctl_fatal('transaction error: %s' % txn_.get_error())
  File "/ryu/ryu/lib/ovs/vsctl.py", line 132, in vsctl_fatal
    raise Exception(msg)        # not call ovs.utils.ovs_fatal for reusability
Exception: transaction error: {"details":"Table Bridge column controller row 69da7121-51fa-4d6e-ad24-18ff9c857b87 references nonexistent row 17c4aa64-ed91-4d8a-a905-a137e6969ad9 in table Controller.","error":"referential integrity violation"}

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