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.

Conflicts:
	functions-common
	lib/ceilometer
	lib/keystone
	lib/neutron
	lib/sahara
	lib/zaqar
	stack.sh

Change-Id: Ia3843818014f7c6c7526ef3aa9676bbddb8a85ca
(cherry picked from commit 2f27a0ed3c609bfcd6344a55c121e56d5569afc9)
Merge "Replace screen_it() with run_process() throughout" into stable/icehouse