Clean up provider usage and definition in XML config

We are very inconsistent about how we're defining and using providers in the XML configuration. We need to meet the following requirements:

• Provide a single top-level element from the substitution group abstract-providers that specifies the providers to use in the absence of any other configuration; default to use-system-providers
• Update the following types (or appropriate enclosing type) to accept an optional nested abstract-providers override:
  • key-store-type
  • credential-store-type
  • hashed-password-type
• Update the following types to use an optional provider attribute (not nested element) to allow selection of a specific provider:
  • credential-store-type
  • key-store-type – already done!
  • hashed-password-type