Add support for HAProxy L7 checks

This change add several configuration options to enable HTTP checks
to the HAProxy configuration, instead of the default TCP connection
checks (which continue to be the default)

Closes-Bug: #1880610
Change-Id: Id42370fe07a2bcbcf28993d44f943d2b5c142107

config.yaml

@@ -95,6 +95,26 @@ options:
     description: |
        Connect timeout configuration in ms for haproxy, used in HA
        configurations. If not provided, default value of 9000ms is used.
+  haproxy-check-http:
+    type: boolean
+    default: False
+    description: |
+      If True uses HTTP (Layer 7) checks for backends in haproxy, used in HA
+      configurations. If False uses a TCP check (default).
+  haproxy-check-request:
+    type: string
+    default: option httpchk GET /healthcheck
+    description: |
+      Enables HTTP protocol to check on the servers health: option httpchk <method> <uri>.
+      This config is used when haproxy-check-http is enabled.
+      See https://www.haproxy.com/documentation/hapee/latest/onepage/#4-option%20httpchk
+  haproxy-check-expected:
+    type: string
+    default: http-check expect status 200
+    description: |
+      Make HTTP health checks consider response contents or specific status codes.
+      This config is used when haproxy-check-http is enabled.
+      See https://www.haproxy.com/documentation/hapee/latest/onepage/#http-check%20expect
   dns-ha:
     type: boolean
     default: False