tests: make functional_tests.sh configurable (#49)

To be able to run `functional_tests.sh` with different parameters, make
it configurable by environment variables, like `RG`, `LOCATION`,
`VM_SIZE`.

For example, it is now possible to simply run like below, without having
to manually update variables in the script:

```
RG="my-test-azinit" LOCATION="germanywestcentral" VM_SIZE="Standard_DS3_v2" make e2e-test
```

Also update the default RG name to `e2etest-azinit`, VM_NAME to
`AzInitFunctionalTest`.

## Testing done

Tested in Azure.