UTs: Disable auto deletion of ports/subnets/nets

Change the behavior of the unit test context managers
for ports, networks, and subnets to skip the deletion
of the objects they create.

Adds explicit delete calls to the tests that require
deletion of the objects.

This is the first patch in a series to eliminate the
automatic deletion of objects created during tests to
improve test performance and make testing more explicit.

Implements: blueprint remove-unit-test-autodeletion
Change-Id: Ifc4490b121310ad64e18936e6fd94a2c9a1e36f2
Merge "UTs: Disable auto deletion of ports/subnets/nets"