Generate interfaces reference doc

Add gen_ref in conf.py to generate reference doc to avoid maintaining
these boilerplates.

Change-Id: I6683c41d39685dcb7f7a619a36210a987c9e9825
Enable pep8 check for config.py in doc

Directory doc is excluded from pep8 check in tox.ini, since we
modified file doc/source/config.py, we should enable the check.

Change-Id: I057f1e6cb1c5f9a6a07043056078e4475f66f288
Added v3 interfaces in reference doc

Added hosts, agents, flavors and so on into reference doc

Change-Id: I32198a0e65551f6ea3c67b72a8d7c6dff7da9f91
Adds quota usage support for the V3 API

Adds the ability to do a detailed query from the
os-quota-sets V3 API extension. This was an interface
added in V3 to allow for the querying of current quota
usage. Adds a shell command quota-usage which
uses this interface to show current quota usage as well
as the limit.

The absolute limits response was removed from
the limits extension in a9e29b7e9fbe14ab42f24802a6d2a457a6317ba3
and so the corresponding novaclient command is also removed.
The "quota-show-usage" command now shows the same information.

Differences between the V2 and V3 API are described here:
https://wiki.openstack.org/wiki/NovaAPIv2tov3

Partially implements blueprint v3-api

Change-Id: I5db72d42120f4ad7a86fbfce20382988f6bbf5d3
Merge "Adds quota usage support for the V3 API"
Merge "Generate interfaces reference doc"
Merge "Enable pep8 check for config.py in doc"
Merge "Added v3 interfaces in reference doc"