Install psutil required by tools/mlock_report.py script

'tools/mlock_report.py' script requires 'psutil' package to be
installed. This ensures it is done before memory_peak service is
started.

Partial-Bug: #1860753
Change-Id: I7b2b6eaf9856c6057e1a4a0054d15074150a6cb6

lib/dstat

@@ -9,6 +9,7 @@
 
 # ``stack.sh`` calls the entry points in this order:
 #
+# - install_dstat
 # - start_dstat
 # - stop_dstat
 
@@ -16,6 +17,14 @@
 _XTRACE_DSTAT=$(set +o | grep xtrace)
 set +o xtrace
 
+# install_dstat() - Install prerequisites for dstat services
+function install_dstat {
+    if is_service_enabled memory_tracker; then
+        # Install python libraries required by tools/mlock_report.py
+        pip_install_gr psutil
+    fi
+}
+
 # start_dstat() - Start running processes
 function start_dstat {
     # A better kind of sysstat, with the top process per time slice

stack.sh

@@ -867,6 +867,13 @@ if is_service_enabled tls-proxy; then
     init_cert
 fi
 
+# Dstat
+# -----
+
+# Install dstat services prerequisites
+install_dstat
+
+
 # Check Out and Install Source
 # ----------------------------