Move database configuration to config file instead of hardcoding

Use:
``` c#
services.AddDbContext<MyHotelDbContext>(options => options.UseSqlServer(Configuration["ConnectionStrings:MyHotelDb"]));
```

And define **appsettings .Development.json** like:
``` js
"ConnectionStrings": {
    "MyHotelDb": "Data Source=(localdb)\\MSSQLLocalDB;Initial Catalog=MyHoteDbl;Trusted_Connection=True;MultipleActiveResultSets=true"
  },
```