remove default=None for config options

In the cfg module default=None is set as the default value, there are a 
lot of StrOpts but few with default=None, to be more unify and clear, we 
should remove it.

Change-Id: I2a5dab763fe9cc07e6c1dbed4628e9cf2ca28bcd
Closes-Bug: #1323975

ironic/common/utils.py

@@ -43,7 +43,6 @@ utils_opts = [
                help='Path to the rootwrap configuration file to use for '
                     'running commands as root.'),
     cfg.StrOpt('tempdir',
-               default=None,
                help='Explicitly specify the temporary working directory.'),
 ]
 

ironic/conductor/manager.py

@@ -71,7 +71,6 @@ LOG = log.getLogger(__name__)
 
 conductor_opts = [
         cfg.StrOpt('api_url',
-                   default=None,
                    help=('URL of Ironic API service. If not set ironic can '
                          'get the current value from the keystone service '
                          'catalog.')),