New list config option to list network features without API extension

Historically Neutron had API extensions to make it clear what feature
is available and what is not. There are however features without new
API extensions, see [1] and [2].
To avoid having lots of config options that makes it possible in a
branchless tempest to detect if a given feature is available or not on
a given Neutron branch this patch proposes a list config option similar
to api_extensions: available_features.
available_features is a list option with the special all entry, and new
tests that depend on API extensionless features can use this to be
skipped on older branches.

[1] https://review.opendev.org/750355
[2] https://review.opendev.org/743695

Change-Id: I9d4be68db0f18dcaded4bfe78f5a314cb450987a

releasenotes/notes/network_feature_enabled_available_features-35f9ac5f253e2ca3.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    New config option to ``network-feature-enabled``: ``available_features``.
+    This is a list which can contain features that are not discoverable
+    through Neutron API, or it can be the special entry ``all``.

tempest/common/utils/__init__.py

@@ -128,3 +128,18 @@ def is_extension_enabled(extension_name, service):
     if extension_name in config_dict[service]:
         return True
     return False
+
+
+def is_network_feature_enabled(feature_name):
+    """A function that will check the list of available network features
+
+    """
+    list_of_features = CONF.network_feature_enabled.available_features
+
+    if not list_of_features:
+        return False
+    if list_of_features[0] == 'all':
+        return True
+    if feature_name in list_of_features:
+        return True
+    return False

tempest/config.py

@@ -790,6 +790,13 @@ NetworkFeaturesGroup = [
                      "entry all which indicates every extension is enabled. "
                      "Empty list indicates all extensions are disabled. "
                      "To get the list of extensions run: 'neutron ext-list'"),
+    cfg.ListOpt('available_features',
+                default=['all'],
+                help="A list of available network features with a special "
+                     "entry all that indicates every feature is available. "
+                     "Empty list indicates all features are disabled."
+                     "This list can contain features that are not "
+                     "discoverable through API."),
     cfg.BoolOpt('ipv6_subnet_attributes',
                 default=False,
                 help="Allow the execution of IPv6 subnet tests that use "