[nginx] define resolver directive in the config

so cosockets use internal resolver and don't have to
go through APIcast resolver.

Closes #488

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - New policy chains system. This allows users to write custom policies to configure what Apicast can do on each of the Nginx phases [PR #450](https://github.com/3scale/apicast/pull/450)
 - Resolver can resolve nginx upstreams [PR #478](https://github.com/3scale/apicast/pull/478)
+- Add `resolver` directive in the nginx configuration [PR #508](https://github.com/3scale/apicast/pull/508)
 - Calls 3scale backend with the 'no_body' option enabled. This reduces network traffic in cases where APIcast does not need to parse the response body [PR #483](https://github.com/3scale/apicast/pull/483)
 - Methods to modify policy chains [PR #505](https://github.com/3scale/apicast/pull/505)
 - Ability to load several environment configurations [PR #504](https://github.com/3scale/apicast/pull/504)

gateway/conf/nginx.conf.liquid

@@ -44,6 +44,11 @@ http {
 
   lua_package_path ";;{{prefix}}/?.lua;{{prefix}}/src/?.lua";
 
+  {% if nameservers %}
+    resolver {{ nameservers | join: " " }};
+  {% endif %}
+
+
   {% for file in "http.d/*.conf" | filesystem %}
     {% include file %}
   {% endfor %}

gateway/src/apicast/cli/environment.lua

@@ -14,10 +14,32 @@ local assert = assert
 local error = error
 local print = print
 local pairs = pairs
+local ipairs = ipairs
+local tostring = tostring
 local insert = table.insert
 local concat = table.concat
 local re = require('ngx.re')
 
+local function parse_nameservers()
+    local resolver = require('resty.resolver')
+    local nameservers = {}
+
+    for _,nameserver in ipairs(resolver.init_nameservers()) do
+        -- resty.resolver returns nameservers as tables with __tostring metamethod
+        -- unfrotunately those objects can't be joined with table.concat
+        -- and have to be converted to strings first
+        insert(nameservers, tostring(nameserver))
+    end
+
+    -- return the table only if there are some nameservers
+    -- because it is way easier to check in liquid and `resolver` directive
+    -- has to contain at least one server, so we can skip it when there are none
+    if #nameservers > 0 then
+        return nameservers
+    end
+end
+
+
 local _M = {}
 ---
 -- @field default_environment Default environment name.
@@ -26,9 +48,11 @@ _M.default_environment = 'production'
 
 --- Default configuration.
 -- @tfield ?string ca_bundle path to CA store file
+-- @tfield ?{string,...} nameservers list of nameservers
 -- @table environment.default_config default configuration
 _M.default_config = {
     ca_bundle = resty_env.value('SSL_CERT_FILE'),
+    nameservers = parse_nameservers(),
 }
 
 local mt = { __index = _M }

gateway/src/resty/resolver.lua

@@ -115,6 +115,8 @@ function _M.init_nameservers(path)
     ngx.log(ngx.INFO, 'adding ', search[i], ' as search domain')
     insert(_M.search, search[i])
   end
+
+  return nameservers
 end
 
 function _M.nameservers()