mgr/orchestrator: don't have _select_orchestrator raise exceptions

mgr/orchestrator: don't have _select_orchestrator raise exceptions

This is problematic as this usually gets invoked via the Mgr remote
method, which will clobber any exception with a RuntimeError.

Instead, just have it return None if one isn't configured, and have the
callers raise the exception if necessary.

Signed-off-by: Jeff Layton <jlayton@redhat.com>