Fix inventory vars containing spaces

Some clouds have availability zones with spaces in their
names. Currently zuul generates something like this as ansible
inventory:

node nodepool_region=None (...) nodepool_az=Failure domain 1

This breaks ansible when trying to read the inventory. Quoting the
inventory variable values solves this issue.

Change-Id: I3e97f40986689b3779efc448eb0d5f1db009e796

zuul/executor/server.py

@@ -942,7 +942,7 @@ class AnsibleJob(object):
             for item in self.getHostList(args):
                 inventory.write(item['name'])
                 for k, v in item['host_vars'].items():
-                    inventory.write(' %s=%s' % (k, v))
+                    inventory.write(' %s="%s"' % (k, v))
                 inventory.write('\n')
                 for key in item['host_keys']:
                     keys.append(key)