Clean network scenarios

Remove single-item collections (lists/dicts)
use filtered neutron queries and net_common objects (Deletable objects) in
loadbalancer_basic

Partially Implements: blueprint neutron-advanced-scenarios

Change-Id: I265b7cd152f766a1f6dc63a84af8090f9ec3bc76
Move admin "delete a server" tests to test_servers

"delete a server" tests, which need the admin privilege, exist in
admin's test_servers. Now we have test_delete_server for testing
"delete a server", and it is better to move them into test_delete
_server for knowing the test coverage.

This patch moves admin "delete a server" tests to test_delete_server.

Change-Id: I079541df10ba7869eb5c6793e051cf93993f3972
Merge "Clean network scenarios"
Merge "Move admin "delete a server" tests to test_servers"