Release Octavia 5.1.0 for Train

This release includes changes to the requirements and lower-constraints
file because of the new pip release. It also includes a bug fix where a
new octavia.conf configuration option (default_connection_limit) with a
sane default was added to fix an issue where HAProxy 1.8 consumes much
more memory and crashes the amphora load balancer. Because of these two
points, we are bumping the minor version.

Change-Id: Ib7a951064edb7b8a89bbf9c9ad69f6cd19e2f543

deliverables/train/octavia.yaml

@@ -42,6 +42,10 @@ releases:
     projects:
       - repo: openstack/octavia
         hash: 8c32d2e6024d9cfa3e6ab8bff43f2a261aaee82d
+  - version: 5.1.0
+    projects:
+      - repo: openstack/octavia
+        hash: fd622009e6641e8b9d04fc66f1e878ef452d8d9f
 branches:
   - name: stable/train
     location: 5.0.0.0rc1