Make network resource creation in isolation configurable

This commit adds a helper method to the base test class
set_network_resources(). This method should be called before the super
call in setUpClass and lets you specify which network resources
are created for a test class in tenant isolation. This is useful for
fine tuning each test class so that only the resources that are needed
get created.

Change-Id: I8f66534dbe3f48552ed6cd238ad237c46d7ee60c
Merge "Make network resource creation in isolation configurable"