config options: centralize section "serial_console"

The config options of the "nova.conf" section "serial_console"
got moved to the new central location "nova/conf/serial_console.py".

The way the centralization is done establishes a directed non-cyclic
dependency:

    functional-module (e.g. virt.libvirt.driver.py)
        => nova.conf.__init__.py
            => section_module (e.g. nova.conf.serial_console.py)

Which makes me believe that the "import_opt" calls are not necessary to
ensure that the proper module gets imported under the cover. That's why
I removed these "import_opt" calls. bp centralize-config-options

A subsequent patch will then improve the help texts.

bp centralize-config-options

Change-Id: I597f427455e976892f60262cc823be3946c83e88

nova/conf/__init__.py

@@ -0,0 +1,26 @@
+# Copyright 2015 OpenStack Foundation
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+# This package got introduced during the Mitaka cycle in 2015 to
+# have a central place where the config options of Nova can be maintained.
+# For more background see the blueprint "centralize-config-options"
+
+from oslo_config import cfg
+
+from nova.conf import serial_console
+
+CONF = cfg.CONF
+
+serial_console.register_opts(CONF)