Add experimental warning for Cells

When we merged Cells a while back, we noted that the usage was
experimental but haven't had a good channel for communicating the
continued state to the user. Since we're adding warning messages
to virt drivers, this patch adds one for Cells as well, in the
Manager's init method.

Change-Id: I75b0bedf57250e7701b00f9ce0f60e4775eacfd7

nova/cells/manager.py

@@ -28,7 +28,9 @@ from nova.cells import utils as cells_utils
 from nova import context
 from nova import exception
 from nova import manager
+from nova.openstack.common.gettextutils import _
 from nova.openstack.common import importutils
+from nova.openstack.common import log as logging
 from nova.openstack.common import periodic_task
 from nova.openstack.common import timeutils
 
@@ -50,6 +52,8 @@ CONF = cfg.CONF
 CONF.import_opt('name', 'nova.cells.opts', group='cells')
 CONF.register_opts(cell_manager_opts, group='cells')
 
+LOG = logging.getLogger(__name__)
+
 
 class CellsManager(manager.Manager):
     """The nova-cells manager class.  This class defines RPC
@@ -70,6 +74,12 @@ class CellsManager(manager.Manager):
     target = oslo_messaging.Target(version='1.25')
 
     def __init__(self, *args, **kwargs):
+        LOG.warn(_('The cells feature of Nova is considered experimental '
+                   'by the OpenStack project because it receives much '
+                   'less testing than the rest of Nova. This may change '
+                   'in the future, but current deployers should be aware '
+                   'that the use of it in production right now may be '
+                   'risky.'))
         # Mostly for tests.
         cell_state_manager = kwargs.pop('cell_state_manager', None)
         super(CellsManager, self).__init__(service_name='cells',