Arnold config : Add default color manager

[johnhaddon]

This translates Gaffer's context-based OCIO config into an Arnold color_manager_ocio, taking effect only if the scene doesn't already specify a color manager. This completes the planned OCIO work for Gaffer 1.3, as specified in #5215.

[johnhaddon]

The GafferArnold tests are failing because Arnold 7.1 is using an older OCIO version that doesn't provide the built-in ACES configs. What to do?

1. Drop the second commit and have folks configure the colour manager manually as before, but now with the help of use defaults which match Gaffer by default.
2. Drop Arnold 7.1 support (testing with 7.2 here all seems well) in Gaffer 1.3.
3. Don't register the auto-color-manager adapter for Arnold 7.1.
4. Disable the adapter in the tests somehow.

[murraystevenson]

What to do?

I think I still need to play with this a little more, but given the OCIO version issues we're already seeing here between Arnold 7.1 & 7.2, my gut feeling is veering towards door number 1...

[murraystevenson]

As discussed, let's go with 1 for Gaffer 1.3 and reconsider the automatic color manager creation at a later date.

[johnhaddon]

As discussed, let's go with 1 for Gaffer 1.3

Dropped, and merging...

[johnhaddon]

Perhaps we should reconsider this for 1.4? It's come up a couple of times lately and has been a bit confusing each time...

Edit : By reconsider, I mean bring back the automatic configuration of Arnold's OCIO settings from Gaffer's, without the user needing to make a colour manager.