log output to a file if $cloud is defined

This eliminates the risk of losing crucial debug output in a
long mkcloud run because you forgot to redirect to a file and
the terminal/screen/tmux scrollback buffer wasn't big enough.

If the 'help' argument is passed, it is assumed that logging
is not required.  Otherwise, a warning is emitted if logging
is not possible due to $cloud not being set.

Closes #1191.

scripts/mkcloud

@@ -15,6 +15,15 @@
 # 2. This tool relies on the script qa_crowbarsetup.sh
 # 3. Please 'export' environment variables according to your needs.
 
+if [[ "$cloud" ]]; then
+    : ${log_dir:=/var/log/mkcloud/$cloud}
+    mkdir -p "$log_dir"
+    log_file=$log_dir/`date -Iseconds`.log
+    exec >  >(tee -ia $log_file)
+    exec 2> >(tee -ia $log_file >&2)
+elif ! [[ " $@ " =~ " help " ]]; then
+    echo "WARNING: \$cloud not set, so won't log to a file" >&2
+fi
 
 if [[ $debug_mkcloud = 1 ]] ; then
     set -x