Decouple test configuration and enable running tests against a real backend

Currently, in the test set the test configuration is (partly?) [hardcoded](https://github.com/apache/cloudstack-go/blob/78fa5e496c8a81f2a7527c1cd5a762a9611e27ad/cloudstack/VirtualMachineService_test.go#L29) using endpoint mocking approach with httptest:

```
func TestVirtualMachineService_DeployVirtualMachine(t *testing.T) {
server := httptest.NewServer( ...
```
It would be great to have some decoupling here to provision endpoint configurations in a flexible way, also enabling automated testing against (multiple) real endpoints.