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
2f27a0ed3c