mgr/orchestrator: ignore Liskov substitution principle violation

mgr/orchestrator: ignore Liskov substitution principle violation

Right now, we're violating the Liskov substitution principle by deriving from `Orchestrator` but `process` takes a sub class of `Completion`:
See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides

The idea is to make Orchestrator a type constructor with `CompletionT` as argument, but this is not supported by mypy: https://github.com/python/typing/issues/548

Signed-off-by: Sebastian Wagner <sebastian.wagner@suse.com>
(cherry picked from commit 9923d2a1193c374b9c2e00b5ee69023cb72a9f00)