Use service_available option to detect Neutron

... because nova-network was removed in Ussuri release.

To run basic neutron tests by default, the default value of
[service_available] neutron option is changed to True.

Change-Id: I74cc344a9c1ae2aa523af754d22392869ff059f5
diff --git a/releasenotes/notes/enable-neutron-by-default-57b87a20acc1ac47.yaml b/releasenotes/notes/enable-neutron-by-default-57b87a20acc1ac47.yaml
new file mode 100644
index 0000000..b8722ea
--- /dev/null
+++ b/releasenotes/notes/enable-neutron-by-default-57b87a20acc1ac47.yaml
@@ -0,0 +1,9 @@
+---
+upgrade:
+  - |
+    Default value of the ``[service_available] neutron`` option has been
+    updated from ``False`` to ``True``.
+
+  - |
+    All tests which require network features are now skipped when
+    the ``[service_available] neutron`` option is set to ``False``