[ALL] Fix for Gearman SSL file check

Although options may not be defined in the config file, the keys
are still present in the CONF structure, but with None values.
We need to take that into consideration.

Change-Id: Id1915a55db3ae4f70895c9f94876e318743dcd19

libra/common/options.py

@@ -82,6 +82,8 @@ def check_gearman_ssl_files():
     for key in ['ssl_ca', 'ssl_cert', 'ssl_key']:
         if key in CONF['gearman']:
             fname = CONF['gearman'][key]
+            if fname is None:
+                continue
             if not os.path.exists(fname):
                 raise Exception("Gearman SSL file %s does not exist" % fname)
             if not os.access(fname, os.R_OK):