Replace usage of LazyPluggable by stevedore driver

Stevedore lib support lazy loading, so this change replace
LazyPluggable class by stevedore extentions and remove it

Stevedore support usage of driver property since 0.4 release,
some of Openstack projects like glance is already using it.

Change-Id: I7d384a73861e18cac04e07f75abd60df7f183aa2

nova/ipv6/api.py

@@ -13,8 +13,8 @@
 #    under the License.
 
 from oslo_config import cfg
+from stevedore import driver
 
-from nova import utils
 
 ipv6_backend_opt = cfg.StrOpt('ipv6_backend',
                               default='rfc2462',
@@ -27,9 +27,8 @@ IMPL = None
 
 def reset_backend():
     global IMPL
-    IMPL = utils.LazyPluggable('ipv6_backend',
-               rfc2462='nova.ipv6.rfc2462',
-               account_identifier='nova.ipv6.account_identifier')
+    IMPL = driver.DriverManager("nova.ipv6_backend",
+                                CONF.ipv6_backend).driver
 
 
 def to_global(prefix, mac, project_id):

nova/utils.py

@@ -372,41 +372,6 @@ def get_my_linklocal(interface):
         raise exception.NovaException(msg)
 
 
-class LazyPluggable(object):
-    """A pluggable backend loaded lazily based on some value."""
-
-    def __init__(self, pivot, config_group=None, **backends):
-        self.__backends = backends
-        self.__pivot = pivot
-        self.__backend = None
-        self.__config_group = config_group
-
-    def __get_backend(self):
-        if not self.__backend:
-            if self.__config_group is None:
-                backend_name = CONF[self.__pivot]
-            else:
-                backend_name = CONF[self.__config_group][self.__pivot]
-            if backend_name not in self.__backends:
-                msg = _('Invalid backend: %s') % backend_name
-                raise exception.NovaException(msg)
-
-            backend = self.__backends[backend_name]
-            if isinstance(backend, tuple):
-                name = backend[0]
-                fromlist = backend[1]
-            else:
-                name = backend
-                fromlist = backend
-
-            self.__backend = __import__(name, None, None, fromlist)
-        return self.__backend
-
-    def __getattr__(self, key):
-        backend = self.__get_backend()
-        return getattr(backend, key)
-
-
 def xhtml_escape(value):
     """Escapes a string so it is valid within XML or XHTML.
 

setup.cfg

@@ -170,6 +170,9 @@ nova.api.v3.test_extensions =
     basic = nova.tests.unit.api.openstack.compute.test_plugins.basic:Basic
     microversions = nova.tests.unit.api.openstack.compute.test_plugins.microversions:Microversions
 
+nova.ipv6_backend =
+    rfc2462 = nova.ipv6.rfc2462
+    account_identifier = nova.ipv6.account_identifier
 
 # These are for backwards compat with Havana notification_driver configuration values
 oslo.messaging.notify.drivers =