[SRU] powering multiple nodes errors with max retries

When powering multiple nodes on at once, in different sub-processes. The following error is generated and the node is not powered off or on accordingly.

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/usr/lib/python2.7/dist-packages/provisioningserver/custom_hardware/seamicro.py", line 324, in power_control_seamicro15k_v2
    server.power_on(using_pxe=True)
  File "/usr/lib/python2.7/dist-packages/seamicroclient/v2/servers.py", line 28, in power_on
    self.manager.power_on(self, using_pxe)
  File "/usr/lib/python2.7/dist-packages/seamicroclient/v2/servers.py", line 114, in power_on
    self._action('power-on', server, action_params)
  File "/usr/lib/python2.7/dist-packages/seamicroclient/v2/servers.py", line 219, in _action
    return self.api.client.put(url, body=body)
  File "/usr/lib/python2.7/dist-packages/seamicroclient/client.py", line 165, in put
    return self._cs_request(url, 'PUT', **kwargs)
  File "/usr/lib/python2.7/dist-packages/seamicroclient/client.py", line 155, in _cs_request
    **kwargs)
  File "/usr/lib/python2.7/dist-packages/seamicroclient/client.py", line 141, in _time_request
    resp, body = self.request(url, method, **kwargs)
  File "/usr/lib/python2.7/dist-packages/seamicroclient/client.py", line 115, in request
    **kwargs)
  File "/usr/lib/python2.7/dist-packages/requests/sessions.py", line 383, in request
    resp = self.send(prep, **send_kwargs)
  File "/usr/lib/python2.7/dist-packages/requests/sessions.py", line 486, in send
    r = adapter.send(request, **kwargs)
  File "/usr/lib/python2.7/dist-packages/requests/adapters.py", line 378, in send
    raise ConnectionError(e)
requests.exceptions.ConnectionError: HTTPConnectionPool(host='192.168.220.102', port=80): Max retries exceeded with url: /v2.0/servers/21/0 (Caused by <class 'httplib.BadStatusLine'>: '')

[Impact]
This bug affects the ability of MAAS (and really any other client/user) to power on multiple (at scale) nodes in a Seamicro15000 chassis using their python-seamicroclient APIT, which is what MAAS uses. This prevents MAAS from being able to power manage many nodes at the time.

[Test Case]
1. Install MAAS. (sudo apt-get install maas)
2. Register SM15k. (maas admin node-group probe-and-enlist-hardware <clkuster-uuid) model=seamicro15k mac=<mac> username=<user> password=<pass> power_control=restapi2
3. Power on Many nodes at the time (juju deploy -n 10)
4. Fail.

With the fix:

3. power on many nodes at the time will succeed.

[Regression Potential]
Minimal. This only affects power on/off of nodes. This has been tested against hardware at one of our labs.