Use six.add_metaclass instead of __metaclass__

According to http://docs.openstack.org/developer/hacking/ it should
be preferred to use six.add_metaclass instead of __metaclass__.

Change-Id: Iec6550421bcf54f30952d9dc9f3a398288d6a283
Image uploads should always happen in the background

Sufficiently large image uploads into Glance from Horizon can cause
timeout errors, since the image upload needs to complete before the
view can render, and such activities can cause browser timeouts.
Since the webserver already has the file, and close-to-open semantics
should prevent cleanups from losing any data, just send it in a
background thread, the same way as if you told Horizon to fetch it
from a remote location.

Change-Id: Ia056367032e0d08edf6f36a8e9f900fddba85fdf
Closes-Bug: 1322399
Handle error from neutron extension check in Overview page

Previously some Neutron API calls in get_neutron_limits are outside
try/except and if an error occurs a traceback will be displayed.

Change-Id: I191aecbf3135ddac3b20dbc7e0f22b63aa3837f9
Closes-Bug: #1252878
Merge "Handle error from neutron extension check in Overview page"
Merge "Use six.add_metaclass instead of __metaclass__"
Merge "Image uploads should always happen in the background"