Allow network driver selection override

Expose a new [networking]/driver option to allow overriding the
network driver.

Our use case is that we have multiple external networks and
floating IP pools in our cells environment, which Murano does not
yet allow you to select. This option allows us to move back to
Nova networking where we have code to handle this automatically.

Change-Id: Ic4e806dc72283fce6348ab1e2966ba3a618f91dc
Closes-Bug: #1617096

murano/common/config.py

@@ -201,7 +201,11 @@ networking_opts = [
 
     cfg.StrOpt('network_config_file', default='netconfig.yaml',
                help='If provided networking configuration will be taken '
-                    'from this file')
+                    'from this file'),
+
+    cfg.StrOpt('driver', default=None, choices=['neutron', 'nova'],
+               help='Network driver to use. Options are neutron or nova.'
+                    'If not provided, the driver will be detected.'),
 ]
 
 stats_opts = [

murano/db/services/environments.py

@@ -264,6 +264,11 @@ class EnvironmentServices(object):
 
     @staticmethod
     def get_network_driver(context):
+        driver = CONF.networking.driver
+        if driver:
+            LOG.debug("Will use {} as a network driver".format(driver))
+            return driver
+
         session = auth_utils.get_token_client_session(
             context.auth_token, context.tenant)
         try:

releasenotes/notes/config-network-driver-77c82d151dead620.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - Configuration option added to explicitly select a
+    networking driver rather than auto-detect.