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
Replace screen_it() with run_process() throughout

run_process will use screen if USE_SCREEN=True (the default),
otherwise it will simply start the requested service. Therefore
wherever screen_it used, run_process can be instead.

Where stop_screen was found it has been replaced with stop_process.

A tail_log function has been added which will tail a logfile in a
screen if USE_SCREEN is True.

lib/template has been updated to reflect the use of the new
functions.

When using sg the quoting in run_process gets very complicated.
To get around this run_process and the functions it calls accepts
an optional third argument. If set it is a group to be used with sg.

Change-Id: Ia3843818014f7c6c7526ef3aa9676bbddb8a85ca
Add $TOP_DIR to the path when calling worlddump script

The missing reference to $TOP_DIR cause stack.sh to fail when called
from outside devstack's directory

Change-Id: I7faec7720896e7dcfe60fa87fb417f22c8801eef
Merge "Add $TOP_DIR to the path when calling worlddump script"
Merge "Replace screen_it() with run_process() throughout"
Merge "Update parameter checking with provider network"