Fix swift logserver prefixes.

Use the default logserver prefix from the config when we intend on using
the default logserver prefix.

Change layout logserver_prefix type in voluptuous from int to str.

Change-Id: Ie3fc837894519de9de89f569ac8e83167d5b4224

zuul/launcher/gearman.py

@@ -245,7 +245,7 @@ class Gearman(object):
                 elif self.config.has_option('swift',
                                             'default_logserver_prefix'):
                     swift_instructions['LOGSERVER_PREFIX'] = \
-                        s_config['logserver_prefix']
+                        self.config.get('swift', 'default_logserver_prefix')
 
                 # Create a set of zuul instructions for each instruction-set
                 # given  in the form of NAME_PARAMETER=VALUE

zuul/layoutvalidator.py

@@ -111,7 +111,7 @@ class LayoutSchema(object):
              'expiry': int,
              'max_file_size': int,
              'max_file_count': int,
-             'logserver_prefix': int,
+             'logserver_prefix': str,
              }
 
     job = {v.Required('name'): str,