Makes Walrus an internal backend component with ComponentId: WalrusBackend. Fixes EUCA-8933

Updates the (de)register/configure scripts as well. euca_conf --register/deregister-walrusbackend
Adds some REST bindings back in for listing buckets and objects for debugging the backend directly as an admin.

Removes the full IAM checks for WalrusBackend and only allows the system admin account to make
requests against Walrus. Removes the Walrus SOAP pipelines to remove Walrus S3 SOAP-API support.