Check for nova-compute before running kill

Unconditionally running this can lead to confusing failure output from
kill as the pgrep matches nothing when nova-compute isn't yet running.

Change-Id: I37cb84fe8e0b393f49b8907af16a3e44f82c46a6
diff --git a/tools/worlddump.py b/tools/worlddump.py
index 0a9f810..e4ba02b 100755
--- a/tools/worlddump.py
+++ b/tools/worlddump.py
@@ -112,6 +112,13 @@
 
 def guru_meditation_report():
     _header("nova-compute Guru Meditation Report")
+
+    try:
+        subprocess.check_call(["pgrep","nova-compute"])
+    except subprocess.CalledProcessError:
+        print "Skipping as nova-compute does not appear to be running"
+        return
+
     _dump_cmd("kill -s USR1 `pgrep nova-compute`")
     print "guru meditation report in nova-compute log"