Adding Swift blobstore configuration properties

Source blob storage (stored during bosh upload release) can now utilise Swift interface.

Example usage in BOSH deploy manifest:

```
  blobstore_endpoint: &blobstore_endpoint
    swift_provider: openstack
    openstack_auth_url: http://xxx.xxx.xxx.xxx:5000/v2.0
    openstack_username: <user>
    openstack_api_key: <key>
    openstack_tenant: <tenant>

  blobstore:
    provider: swift
    options:
      <<: *blobstore_endpoint
      container_name: bosh-source-blobs

  compiled_package_cache:
    options:
      <<: *blobstore_endpoint
      container_name: bosh-compiled-package-cache
```