packaging: setup: Fixing websocket proxy key extraction

As for other keys, extract websocket proxy key out of a
transaction so it would be already available before the
websocket proxy misc plugin.

Change-Id: Id8a608383252b50a4d76f73b0bbbe8c3f6c9a692
Signed-off-by: Simone Tiraboschi <stirabos@redhat.com>