Testcase to create bulk networks in one API call

Added "test_bulk_create_delete_network" test case in the
test_networks.py script of "api/networks" tests.
Also added supporting method "create_bulk_network" to
network_client.py.

Change-Id: I8325caf5405714686fa2e78c7c1589854e271860
Implements: blueprint quantum-test-extended-coverage
2 files changed