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/zaqar b/lib/zaqar
index 0d33df2..33f1092 100644
--- a/lib/zaqar
+++ b/lib/zaqar
@@ -162,9 +162,9 @@
 # start_zaqar() - Start running processes, including screen
 function start_zaqar {
     if [[ "$USE_SCREEN" = "False" ]]; then
-        screen_it zaqar-server "zaqar-server --config-file $ZAQAR_CONF --daemon"
+        run_process zaqar-server "zaqar-server --config-file $ZAQAR_CONF --daemon"
     else
-        screen_it zaqar-server "zaqar-server --config-file $ZAQAR_CONF"
+        run_process zaqar-server "zaqar-server --config-file $ZAQAR_CONF"
     fi
 
     echo "Waiting for Zaqar to start..."