[BUGFIX] OmegaConf forward compatibility
Apply updates where OmegaConf is breaking with 2.1.0.

See #2316 for the original PR to v1.0.0.

Note, we have to upper bound OmegaConf as Hydra 1.0.6 is not compatible with OmegaConf 2.1.0. When the new version of Hydra is released we should remove the upper bound.