install pstack on gallium

Jenkins has some performance issue where a thread is deadlocked eating
100% CPU.  That makes all the API calls very slow and cascade to Zuul
waiting for Jenkins and not processing anything.

pstack might give clue about what is going on in the thread by getting a
system stacktrace.

The performance issue is tracked by bug 48025.

Change-Id: Ie33e1adb8d9facab8c57376cdd418baf0d8a208a