do not encode some viz attributes

[graphaelli]

unpack_dashboards will now handle it through make update

fixes #1017

[graphaelli]

tested this manually by:

confirm bug:

make update apm-server
ln -s _meta/kibana.generated
./apm-server -E setup.dashboards.enabled=true

open dashboards, see they don't load. confirm saved objects are double json encoded

confirm fix:
repeat with this change and confirm all is well.

This would be a good one for https://github.com/elastic/apm-integration-testing.

[simitt]

Thanks for taking care of this!