Custom DB configuration

[peppelinux]

We can configure the storage for client_Db, sso_db and session_db

op:
  client_db:
    class: oidcendpoint.in_memory_db.InMemoryDataBase
  session_db:
    class: oidcendpoint.in_memory_db.InMemoryDataBase
  sso_db:
    class: oidcendpoint.sso_db.SSODb

This commit is needed to permit other developers to adopt oidcendpoint in their framework and overcome the inMemoryDataBase

[rohe]

I used to have more then server_info below op in the configuration file. Now server_info is the only component of op. So we should probably get rid of the hierarchy and just have op or server_info.