Ensure unit tests do not leak resources
test_destroy_volume(): Do not use volume from fixture

Otherwise cleanup will fail when trying to destroy a volume that does
not exist.
Test suite: Fail if left-overs found
test_volumes.test_detach_unattached(): Remove unneeded check
Remove test suite leak
Use node fixture
Sample volume: Detach before destroying
Fix test
Simplify test
More robust network tests
Reuse fixtures
Shorten function name
Fix node leaks in test
Fix test
Remove superfluous test
VagrantNetwork.allocated: Always return values from catalogue.

So that we don't leak network objects in unit tests.
Fix test
Remove dead code
Tests wrap-up: Only fail if active networks left over.
Revisit network cleanup in unit tests.