Switch to python3 for memory_peak service

When starting 'memory_peak' service is using python command instead of
python3, while psutil (required package) is most probably being
installed into the python3 environment (as we are dropping python2.7
support).

Closes-Bug: #1860753
Change-Id: Ia2b7e2e33d784560443131e2965f520b361a54e3
diff --git a/tools/memory_tracker.sh b/tools/memory_tracker.sh
index 63f25ca..6c36534 100755
--- a/tools/memory_tracker.sh
+++ b/tools/memory_tracker.sh
@@ -14,7 +14,7 @@
 
 set -o errexit
 
-PYTHON=${PYTHON:-python}
+PYTHON=${PYTHON:-python3}
 
 # time to sleep between checks
 SLEEP_TIME=20