Remove DbConfig and new constructors on DbContextBase and refactor Migrations to use explcit generation of IOqtaneDatabase instance

Oqtane.Shared/Shared/InstallConfig.cs

@@ -1,9 +1,29 @@
+using System;
+using Oqtane.Interfaces;
+
 namespace Oqtane.Shared
 {
     public class InstallConfig
     {
+        private IOqtaneDatabase _database;
+
         public string ConnectionString { get; set; }
         public string DatabaseType { get; set; }
+
+        public IOqtaneDatabase Database
+        {
+            get
+            {
+                if (_database == null)
+                {
+                    var type = Type.GetType(DatabaseType);
+                    _database = Activator.CreateInstance(type) as IOqtaneDatabase;
+                }
+
+                return _database;
+            }
+        }
+
         public string Aliases { get; set; }
         public string TenantName { get; set; }
         public bool IsNewTenant { get; set; }