Error `[json.exception.out_of_range.401] array index 2 is out of range`

[cassava]

Steps to reproduce:

cd plugins/vtd/tests
cloe-launch shell -P profile_default.py
cloe-engine -l debug run test_vtd_smoketest.json bootstrap_vtd.json ../../../tests/option_performance_measurements.json

[tobifalk]

There are in fact two issues:

1. The environment variable ${CLOE_ROOT} used in the stackfile is currently only defined in the bats setup. Could we maybe define it in the profile's package_info() method?
2. Issue 1. causes an exception thrown here. The variable interpolation is invoked from inside the loop over the stack files to be merged (s. recursive call here and here). When the exception is caught here, the index i value causes the out-of-range error.

[cassava]

Thanks for your root-cause analysis @tobifalk.

So I figured out a way to define some environment variables in #92. Haven't gotten a chance to look at the root cause.