Only format swift string parameters

Some swift parameters are integer in type and they do not support the
string format operation. Only format parameters if they are a string and
pass integers (and other types) through as is.

Change-Id: I8d7a5bb25f7be00a5cb02b81292a548f033cad8a

zuul/launcher/gearman.py

@@ -231,6 +231,8 @@ class Gearman(object):
                 s_config = {}
                 s_config.update((k, v.format(item=item, job=job,
                                              change=item.change))
+                                if isinstance(v, basestring)
+                                else (k, v)
                                 for k, v in s.items())
 
                 (swift_instructions['URL'],