New update mechanism for Azure certs (#78)

* Fixes problem with trying to update certs which are in use
* Transparently treats slots the same way as normal apps
* Fixes problem with base app settings being changed rather than slot settings
* Refactors very large PersistAsync method into parts

src/FluffySpoon.AspNet.LetsEncrypt.Azure/AzureAppInstance.cs

@@ -0,0 +1,109 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Microsoft.Azure.Management.AppService.Fluent;
+using Microsoft.Azure.Management.AppService.Fluent.Models;
+using Microsoft.Azure.Management.Fluent;
+
+namespace FluffySpoon.AspNet.LetsEncrypt.Azure
+{
+    /// <summary>
+    /// A reference to an Azure app, which might be either a simple app, or a deployment slot
+    /// </summary>
+    public class AzureAppInstance
+    {
+        readonly IWebApp _app;
+        readonly IDeploymentSlot _slot;
+
+        public AzureAppInstance(IWebApp app)
+        {
+            _app = app;
+        }
+
+        public AzureAppInstance(IWebApp app, IDeploymentSlot slot)
+        {
+            _app = app;
+            _slot = slot;
+        }
+
+        /// <summary>
+        /// A general purpose name for this instance
+        /// </summary>
+        public string DisplayName => IsSlot ? $"{_app.Name}/{_slot.Name}" : _app.Name;
+
+        /// <summary>
+        /// Get the host names associated with this instance
+        /// </summary>
+        public ISet<string> HostNames => IsSlot ? _slot.HostNames : _app.HostNames;
+
+        private bool IsSlot => _slot != null;
+
+        public Task<HostNameBindingInner> GetHostNameBindingAsync(IAzure client, string resourceGroupName, string domain)
+        {
+            if (IsSlot)
+            {
+                return client.WebApps.Inner.GetHostNameBindingSlotAsync(resourceGroupName,
+                    _app.Name,
+                    _slot.Name,
+                    domain);
+            }
+            else
+            {
+                return client.WebApps.Inner.GetHostNameBindingAsync(resourceGroupName,
+                    _app.Name,
+                    domain);
+            }
+        }
+
+        public Task SetHostNameBindingAsync(IAzure client, string resourceGroupName, string domain, HostNameBindingInner binding)
+        {
+            if (IsSlot)
+            {
+                return client.WebApps.Inner.CreateOrUpdateHostNameBindingSlotWithHttpMessagesAsync(
+                    resourceGroupName,
+                    _app.Name,
+                    domain,
+                    binding,
+                    _slot.Name);
+            }
+            else
+            {
+                return client.WebApps.Inner.CreateOrUpdateHostNameBindingWithHttpMessagesAsync(
+                    resourceGroupName,
+                    _app.Name,
+                    domain,
+                    binding
+                );
+            }
+        }
+
+        public Task<IReadOnlyDictionary<string, IAppSetting>> GetAppSettings()
+        {
+            if (IsSlot)
+            {
+                return _slot.GetAppSettingsAsync();
+            }
+            else
+            {
+                return _app.GetAppSettingsAsync();
+            }
+        }
+
+        public Task UpdateAppSettingAsync(string key, string value)
+        {
+            if (IsSlot)
+            {
+                return _slot.Update()
+                    .WithAppSetting(key, value)
+                    .ApplyAsync();
+            }
+            else
+            {
+                return _app.Update()
+                    .WithAppSetting(key, value)
+                    .ApplyAsync();
+            }
+        }
+
+        public string RegionName => IsSlot ? _slot.RegionName : _app.RegionName;
+    }
+}