Add in check for successful swift setup

If the swift credentials are wrong zuul will crash. This change
adds in a check incase setting up swift access fails. A signed
URL will still be processed but will likely be wrong. A warning
is logged.

Change-Id: I6716e06ef3e42914c90d5857abab799e39d8089d
diff --git a/zuul/lib/swift.py b/zuul/lib/swift.py
index 8f926ad..2cbd05e 100644
--- a/zuul/lib/swift.py
+++ b/zuul/lib/swift.py
@@ -14,6 +14,7 @@
 
 import hmac
 from hashlib import sha1
+import logging
 from time import time
 import os
 import random
@@ -23,6 +24,8 @@
 
 
 class Swift(object):
+    log = logging.getLogger("zuul.lib.swift")
+
     def __init__(self, config):
         self.config = config
         self.connection = False
@@ -35,7 +38,13 @@
                 for x in range(20)
             )
 
-        self.connect()
+        self.storage_url = ''
+
+        try:
+            self.connect()
+        except Exception as e:
+            self.log.warning("Unable to set up swift. Signed storage URL is "
+                             "likely to be wrong. %s" % e)
 
     def connect(self):
         if self.config.has_section('swift'):