Add service to tcpdump during run

This adds a service to run a tcpdump during the run.  This can be
useful to capture various network traffic for post analysis.

There didn't seem to quite be an appropriate place to document it, so
a new debugging file is started, with some terse explaination of our
various system-wide debugging services.

Change-Id: I09aaa57611c5047d09a9bce7932d34e9d50b30e6
diff --git a/stack.sh b/stack.sh
index 022d5b9..dfc9d24 100755
--- a/stack.sh
+++ b/stack.sh
@@ -614,6 +614,7 @@
 source $TOP_DIR/lib/neutron
 source $TOP_DIR/lib/ldap
 source $TOP_DIR/lib/dstat
+source $TOP_DIR/lib/tcpdump
 source $TOP_DIR/lib/etcd3
 
 # Extras Source
@@ -1053,6 +1054,12 @@
 # A better kind of sysstat, with the top process per time slice
 start_dstat
 
+# Run a background tcpdump for debugging
+# Note: must set TCPDUMP_ARGS with the enabled service
+if is_service_enabled tcpdump; then
+    start_tcpdump
+fi
+
 # Etcd
 # -----