More flexible functional test (to cover northbound-only interactions)

[fgalan]

(Comes from #1579 (comment))

Currently functional test use a script base in three fixed steps:

1. Group provision
2. Send measure
3. Check result at CB

Thus, they doesn't allow steps like this one (involving only northbound interactions), inspired in the PR #1579 functionality

1. Create device A
2. Create device B
3. Create device C
4. Do POST /iot/op/delete with A, B
5. Check that only C exist in the IOTA

It would be great to make functional tests more flexible to cover this kind of cases