[JUJU-1198] Rewrite to bash-based nw-destroy-model-lxd

[anvial]

This patch adds destroy model test to the model suite in bash-based tests.

Original (python-based) tests live here.

Checklist

• [x] Code style: imports ordered, good names, simple structure, etc
• [x] Comments saying why design decisions were made
• ~[ ] Go unit tests, with comments saying what you're testing~
• ~[ ] Integration tests, with comments saying what you're testing~
• ~[ ] doc.go added or updated in changed packages~

QA steps

cd tests
./main.sh -v -p lxd model test_model_destroy