title: Get IdentityServer3 to use existing user sql database
body: <p>I am trying to authenticate a user against an existing user database.
  I have an existing database with a users table and I want the Identity Server to look at this database and get authenticate the user.</p>
  
  <p>I have found examples for MembershipReboot and AspNetIdenetity, but I am confused on how I need to create my own user database mapping.</p>
  
  <p>I am not trying to create an external provider e.g Facebook</p>
  
  <p>Has anyone got any ideas or seen any tutorials on how this can be done?</p>