Add end-to-end tests (apply, plan, destroy) for examples.
When run, `tests/examples_e2e`:
1. Create an environment for tests to run (folder, project vpc network)
2. For each marked example (with `e2e` tftest directive), run apply, plan, destroy
3. Verify:
* no failure in apply
* empty plan after apply
* no failure during destroy
4. When all tests are done, destroy test environment
More details in `tests/examples_e2e/README.md`
## net-lb-int
* Fix error on apply of example:
```
Error creating RegionBackendService: googleapi: Error 400: Invalid value for field 'resource.backends[0].balancingMode': 'UTILIZATION'. Balancing mode must be CONNECTION for an INTERNAL backend service., invalid
```
* remove unused `balancing_mode` variable, as only one value is possible anyhow
## net-lb-ext
* update in the `backends` description
## net-lb-proxy-int
* update in the `backends` description
## net-lb-app-int
* added validation of `balancing_mode`
* fixed other validations
## net-lb-app-ext
* added validation of `balancing_mode`
* fixed other validations
* removed validation for `locality_lb_policy` as this variable is not used in this module
Closes: #1767
Ludovico Magnocavallo
Wiktor Niesiobędzki
Luca Prete
Julio Castillo
Miren Esnaola