Add helpers to create/destroy vApp test fixtures

We need to create and tear down vApps in many integration tests for
Core -- Query, Vapp, Vm at least.

The self.create_test_case_vapps method creates a number of vApps in
a given vDC, with bridged networking to a list of networks. These
can have a configurable name prefix, to help distinguish/separate them
from other tests.

The self.delete_vapps takes a list of Vapp objects as returned by
self.create_test_case_vapps and destroys them.
Switch QueryRunner test to use vApp helper
Fix prefix to not require trailing hyphen
Merge pull request #54 from alphagov/vapp_fixture_helper

[#65713646] Add helper methods to create/delete vApps in tests