Schedule coverity less frequently

Only run the coverity build on Tuesday, Thursday, and Saturday (to
comply with rate limiting).

Author: tlyu

buildbot/master.cfg

@@ -98,7 +98,8 @@ c['schedulers'].append(Scheduler(name="master-doc", branch='master',
 
 # Coverity scan
 c['schedulers'].append(Nightly(name="master-coverity", branch='master',
-                               hour=3, builderNames=['master-coverity']))
+                               dayOfWeek=[1, 3, 5], hour=3,
+                               builderNames=['master-coverity']))
 
 ####### BUILDERS