zuul_debug_info: Add /proc/cpuinfo output

There were some jobs recently that showed an unexpected processor count.
Add some data to allow to debug this.

Change-Id: I587a492d1aa94b0886c7e9a2260a3e2eb384e788

roles/validate-host/library/zuul_debug_info.py

@@ -27,6 +27,7 @@ command_map = {
     'network_neighbors': 'ip neighbor show',
     'df_i': 'df -i',
     'df_m': 'df -m',
+    'proc_cpuinfo': 'cat /proc/cpuinfo',
 }