fix(db): assigning non existing sandbox or multiple times the same sandbox #389

Job	Run time
Build wheels	1m 4s
client-generation	3m 20s
Unit test - ./extensions/gubbins/gubbins-core	1m 27s
Unit test - ./extensions/gubbins/gubbins-db	1m 38s
Unit test - ./extensions/gubbins/gubbins-routers	2m 10s
Unit test - ./extensions/gubbins/gubbins-client	1m 44s
Unit test - ./extensions/gubbins/gubbins-cli	1m 51s
build-image	2m 43s
pytest-integration	12m 35s
	28m 32s

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