Split datastores.

Makes no sense trying to combine global stuff and per machine category
stuff into the one set of data stores.

This change will help future changes in how groups are distributed
across may machine categories.

Documentation has been updated to reflect new system.

General changes that might be required include:

* Existing datastores should be based on base.MachineCategoryDataStore
  instead of base.baseDataStore

* Some old methods in MachineCategoryDataStore are no longer used. In
  particular methods that deal with people (as opposed to accounts).
  These should be removed.

* DATASTORES in /etc/karaage/global_settings.conf is now called
  MACHINE_CATEGORY_DATASTORES.

* openldap_group needs to be renamed to openldap_account_group or
  ds389_group needs to be renamed to ds389_account_group in
  MACHINE_CATEGORY_DATASTORES.

Change-Id: Id9777e2d9e995b892b22227c1d66cb98108f593d