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
diff --git a/lib/cinder b/lib/cinder
index 0426dbe..6032a9a 100644
--- a/lib/cinder
+++ b/lib/cinder
@@ -456,7 +456,7 @@
     # Kill the cinder screen windows
     local serv
     for serv in c-api c-bak c-sch c-vol; do
-        screen_stop $serv
+        stop_process $serv
     done
 
     if is_service_enabled c-vol; then