Restrict RetrieveSecret to task authentication

Part of #1540 (closed)

RetrieveSecret was meant to be used by both agents and tasks, but only tasks (well, base-worker) uses it.

I removed a test from project/tests/test_ponos_view.py since it was redundant with the existing tests in ponos/tests/test_api.py. All tests in test_ponos_view.py should be slowly moved to ponos/, since those were for the old Ponos view overrides from before the merge.