PyJobTransforms: Minor AMI tag handling tweaks

By defining new test tags I had troubles with forcing inputs. This MR changes two things:

• If input/output is explicitly set, use it and do not try to override it with a default.
• Ignore legacy JSON inputs/outputs for new tags as those can still be useful for tests.

Can please @mavogel or @graemes look at this as I'm afraid to break anything.

Note that I tested:

• standard q-tests
• new tags recently created that motivated the change (d1609, r12276)
• overriding the outputs works as expected

Cheers, Tadej