Add opt doc and reno for noop interface driver

The `interface_driver` option needs some clarification.
While we only support three interface mechanisms,
it's possible that deployers can use their own
interface driver, so using oslo_config's choices
for this option does not make sense.

Depends-On: I2c3d6e4234bf7185b8da3c8e1701069c3a165ffc
Change-Id: I1cc710dbe87e9e23fbed63dcd5f1017978393c66

manila/share/drivers/service_instance.py

@@ -93,8 +93,18 @@ share_servers_handling_mode_opts = [
     cfg.StrOpt(
         "interface_driver",
         default="manila.network.linux.interface.OVSInterfaceDriver",
-        help="Vif driver. Used only with Neutron and "
+        help="Module path to the Virtual Interface (VIF) driver class. This "
-             "if driver_handles_share_servers=True."),
+             "option is used only by drivers operating in "
+             "`driver_handles_share_servers=True` mode that provision "
+             "OpenStack compute instances as share servers. This option is "
+             "only supported with Neutron networking. "
+             "Drivers provided in tree work with Linux Bridge "
+             "(manila.network.linux.interface.BridgeInterfaceDriver) and OVS "
+             "(manila.network.linux.interface.OVSInterfaceDriver). If the "
+             "manila-share service is running on a host that is connected to "
+             "the administrator network, a no-op driver "
+             "(manila.network.linux.interface.NoopInterfaceDriver) may "
+             "be used."),
     cfg.BoolOpt(
         "connect_share_server_to_tenant_network",
         default=False,

releasenotes/notes/generic-driver-noop-interface-driver-24abcf7af1e08ff9.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    A "no-op" interface driver (manila.network.linux.interface.NoopInterfaceDriver)
+    has been introduced to work with drivers that create and manage
+    lifecycle of share servers (``driver_handles_share_servers=True``)
+    through service instance virtual machines using OpenStack Compute. This
+    interface driver can be used when manila-share is running on a machine
+    that has access to the administrator network used by Manila.