Add subnet_id option

When tested network contained more than one subnet then test failed
due to assert call.

Subnet_id option specifies which subnet should be used for testing
when more than one subnet in network is present. If subnet_id is
specified then each created VM is assigned a floating IP within
the range of the subnet.

Closes-Bug: 1856671
Change-Id: Ie4d44b16595055201e12f10982fc78683a12d41a
diff --git a/releasenotes/notes/add-subnet-id-config-option-fac3d6f12abfc171.yaml b/releasenotes/notes/add-subnet-id-config-option-fac3d6f12abfc171.yaml
new file mode 100644
index 0000000..a1bd4c5
--- /dev/null
+++ b/releasenotes/notes/add-subnet-id-config-option-fac3d6f12abfc171.yaml
@@ -0,0 +1,8 @@
+---
+features:
+  - A new config option 'subnet_id' is added to section
+    'network' to specify subnet which should be used for
+    allocation of IPs for VMs created during testing.
+    It should be used when the tested network contains more
+    than one subnet otherwise test of external connectivity
+    will fail. (Fixes bug #1856671)