In cleaning up functional tests for nova-net, I discovered some
problems in network create:
* --subnet option is required in network create command
* Switch API to use /os-networks rather than /os-tenant-networks as this
  is what we were actually using via novaclient
* Fix functional tests for nova-net
* Normalize some private function names in network/v2/network.py

Change-Id: I426b864406756d58d140575a3a45ee9aee67ce84

In volume snapshot command, <volume> is the same
as <snapshot-name> when --volume is not specified,
but <volume> cannot be None, so when <snapshot-name>
is not specified (<snapshot-name> is None), a NoneType
error appears.
So make <snapshot-name> no longer optional, it should
be always present.

Change-Id: I3d9f10753a8ef601e70816421c160598e2cc811f
Closes-bug: #1659894

The patch https://review.openstack.org/#/c/198506/ rename option
--os-endpoint-type to --os-interface a year ago, but lots of users
switch to osc from these project specified clients, like: novaclient
and neutronclient, they used OS_ENDPOINT_TYPE and --os-endpoint-type
for long time, we should add the notes into openstackclient manpage,
let them know the gap exists, and how to make it works.

Change-Id: Ic84a60744aa4e519994a18104deae0c2b5b8b0ed
Closes-Bug: #1678144

--nic option is quite unhandy. It is better to have
two seperate options --network and --port to add a
network to a new server.

Change-Id: I523abdc83ca2dd4c5dd3871f8f109c2bf57c2e02
Closes-Bug: #1612898

When router_distributed=True is set in neutron.conf,
user can't override this and create a centralized router
through openstack client. Openstack client allows modifying
routers from distributed to centralized after creation but
not supporting centralized flag during creation. We allow
centralized and distributed flags during router creation
with this change.

Closes-bug: #1664255
Change-Id: I75f72ca695338ad8c381cfa89fbb9d8e61ee7bc5