Restructure Barbican plugins to accommodate all use cases

Barbican has need to discover, integrate and manage several types of plugins, and the current project file structure doesn't easily house them all. Plugins include the current HSM style 'crypto' plugins with their encrypt/decrypt/kek-meta behaviors, as well as the emerging key-storage plugins such as Dogtag and KMIP. Other plugin types include order and workflow processing (such as for SSL certificate generation and provisioning) and supporting adapters for event notifications. Hence this blueprint calls for restructuring the Barbican project to find a 'home' for each of these types, both for their abstract interface contracts as well as for default implementations.