adopt to renamed "struct metadata::TaskContext"

Related to [this](https://github.com/rock-core/tools-orogen_metadata/pull/2) PR.

What about the [make_orogen_metadata_optional](https://github.com/rock-core/base-orogen-std/tree/make_orogen_metadata_optional)? Its in rock1408, but not in master? And commit-msg states it should not be released...? *scratch*

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>