Update parameter checking with provider network

Currently, devstack can't deploy OpenStack with flat provider network.

devstack checks whether SEGMENTATION_ID is provided or not when it
deploys openstack with provider network. If it is not provided devstack
prints a error message and dies.
On the other hand, devstack also gets an error from "neutron net-create"
command even if it sets SEGMENTATION_ID with flat option.

In addition to that, because neutron allows partial specs these days,
SEGMENTATION_ID checking should be removed completely.

Also, devstack uses PHYSICAL_NETWORK both as "network name" and "physical
network name" in neutron command. If it is not provided, the command fails
to create network. So devstack should check whether PHYSICAL_NETWORK is
provided.

See also:
https://blueprints.launchpad.net/neutron/+spec/provider-network-partial-specs

Change-Id: I2273e3369c688bbce1a6e39b43635b5548cc2b98
Closes-Bug: #1362382
Merge "Update parameter checking with provider network"