default=None is unneeded in config definitions

It's not required for setting config options, as it's the default, and
there were only 2 final places where it was set in the code, so we
should just clear that out.

Change-Id: I33dd0c002e48a5f5d26047b7f87643fce67f9e64
Closes-Bug: #1323975

nova/virt/libvirt/imagebackend.py

@@ -66,7 +66,6 @@ __imagebackend_opts = [
                default='',  # default determined by librados
                help='Path to the ceph configuration file to use'),
     cfg.StrOpt('hw_disk_discard',
-               default=None,
                help='Discard option for nova managed disks (valid options '
                     'are: ignore, unmap). Need Libvirt(1.0.6) Qemu1.5 '
                     '(raw format) Qemu1.6(qcow2 format)'),

nova/volume/cinder.py

@@ -61,7 +61,7 @@ cinder_opts = [
                help='Number of cinderclient retries on failed http calls',
             deprecated_group='DEFAULT',
             deprecated_name='cinder_http_retries'),
-    cfg.IntOpt('http_timeout', default=None,
+    cfg.IntOpt('http_timeout',
                help='HTTP inactivity timeout (in seconds)',
                deprecated_group='DEFAULT',
                deprecated_name='cinder_http_timeout'),