Add ConfigAwarePlaceholders class (#997)

* Add ConfigAwarePlaceholders class

This allows placeholder values to be sourced from IConfiguration in
addition to set values.

* Rework how IPlaceholders is decorated in container

test/Ocelot.UnitTests/Infrastructure/ConfigAwarePlaceholdersTests.cs

@@ -0,0 +1,78 @@
+namespace Ocelot.UnitTests.Infrastructure
+{
+    using System;
+    using Moq;
+    using Ocelot.Infrastructure;
+    using Responses;
+    using Shouldly;
+    using Microsoft.Extensions.Configuration;
+    using Xunit;
+
+    public class ConfigAwarePlaceholdersTests
+    {
+        private readonly IPlaceholders _placeholders;
+        private readonly Mock<IPlaceholders> _basePlaceholders;
+
+        public ConfigAwarePlaceholdersTests()
+        {
+            var configurationBuilder = new ConfigurationBuilder();
+            configurationBuilder.AddJsonFile("appsettings.json");
+            var configuration = configurationBuilder.Build();
+
+            _basePlaceholders = new Mock<IPlaceholders>();
+            _placeholders = new ConfigAwarePlaceholders(configuration, _basePlaceholders.Object);
+        }
+
+        [Fact]
+        public void should_return_value_from_underlying_placeholders()
+        {
+            var baseUrl = "http://www.bbc.co.uk";
+            const string key = "{BaseUrl}";
+            
+            _basePlaceholders.Setup(x => x.Get(key)).Returns(new OkResponse<string>(baseUrl));
+            var result = _placeholders.Get(key);
+            result.Data.ShouldBe(baseUrl);
+        }
+
+        [Fact]
+        public void should_return_value_from_config_with_same_name_as_placeholder_if_underlying_placeholder_not_found()
+        {
+            const string expected = "http://foo-bar.co.uk";
+            var baseUrl = "http://www.bbc.co.uk";
+            const string key = "{BaseUrl}";
+            
+            _basePlaceholders.Setup(x => x.Get(key)).Returns(new ErrorResponse<string>(new FakeError()));
+            var result = _placeholders.Get(key);
+            result.Data.ShouldBe(expected);
+        }
+
+        [Theory]
+        [InlineData("{TestConfig}")]
+        [InlineData("{TestConfigNested:Child}")]
+        public void should_return_value_from_config(string key)
+        {
+            const string expected = "foo";
+            
+            _basePlaceholders.Setup(x => x.Get(key)).Returns(new ErrorResponse<string>(new FakeError()));
+            var result = _placeholders.Get(key);
+            result.Data.ShouldBe(expected);
+        }
+
+        [Fact]
+        public void should_call_underyling_when_added()
+        {
+            const string key = "{Test}";
+            Func<Response<string>> func = () => new OkResponse<string>("test)");
+            _placeholders.Add(key, func);
+            _basePlaceholders.Verify(p => p.Add(key, func), Times.Once);
+        }
+
+        [Fact]
+        public void should_call_underyling_when_removed()
+        {
+            const string key = "{Test}";
+            _placeholders.Remove(key);
+            _basePlaceholders.Verify(p => p.Remove(key), Times.Once);
+        }
+    }
+}