implement canary annotation and alternative backends

Adds the ability to create alternative backends. Alternative backends enable
traffic shaping by sharing a single location but routing to different
backends depending on the TrafficShapingPolicy defined by AlternativeBackends.

When the list of upstreams and servers are retrieved, we then call
mergeAlternativeBackends which iterates through the paths of every ingress
and checks if the backend supporting the path is a AlternativeBackend. If
so, we then iterate through the map of servers and find the real backend
that the AlternativeBackend should fall under. Once found, the
AlternativeBackend is embedded in the list of VirtualBackends for the real
backend.

If no matching real backend for a AlternativeBackend is found, then the
AlternativeBackend is deleted as it cannot be backed by any server.

rootfs/etc/nginx/lua/balancer.lua

@@ -122,9 +122,57 @@ local function sync_backends()
   end
 end
 
+local function route_to_alternative_balancer(balancer)
+  if not balancer.alternative_backends then
+    return false
+  end
+
+  -- TODO: support traffic shaping for n > 1 alternative backends
+  local alternative_balancer = balancers[balancer.alternative_backends[1]]
+
+  local clean_target_header = util.replace_special_char(alternative_balancer.traffic_shaping_policy.header, "-", "_")
+
+  local header = ngx.var["http_" .. clean_target_header]
+  if header then
+    if header == "always" then
+      return true
+    elseif header == "never" then
+      return false
+    end
+  end
+
+  local clean_target_cookie = util.replace_special_char(alternative_balancer.traffic_shaping_policy.cookie, "-", "_")
+
+  local cookie = ngx.var["cookie_" .. clean_target_cookie]
+  if cookie then
+    if cookie == "always" then
+      return true
+    elseif cookie == "never" then
+      return false
+    end
+  end
+
+  if math.random(100) <= alternative_balancer.traffic_shaping_policy.weight then
+    return true
+  end
+
+  return false
+end
+
 local function get_balancer()
   local backend_name = ngx.var.proxy_upstream_name
-  return balancers[backend_name]
+
+  local balancer = balancers[backend_name]
+  if not balancer then
+    return
+  end
+
+  if route_to_alternative_balancer(balancer) then
+    local alternative_balancer = balancers[balancer.alternative_backends[1]]
+    return alternative_balancer
+  end
+
+  return balancer
 end
 
 function _M.init_worker()