Add managing for nova-serialproxy service

By default service nova-serialproxy should be disabled.
To skip disabling add next data to the model:

    nova:
      controller:
        ....
        serial_proxy:
          enabled: True

PROD-36897

Change-Id: I8a309594d76e32c0165cda165e19564a02c28dd2
diff --git a/README.rst b/README.rst
index 3333007..15a48c0 100644
--- a/README.rst
+++ b/README.rst
@@ -1606,3 +1606,13 @@
       controller:
         filter_scheduler:
           max_instances_per_host: 50
+
+Enable Nova nova-serialproxy service:
+
+.. code-block:: yaml
+
+    nova:
+      controller:
+        ....
+        serial_proxy:
+          enabled: True