Refactor cross_tenant to security_groups_basic_ops

Name is more appropriate.

Also - decouple cross_tenant from in-tenant test
With some cosmetic changes to the code.

Partially Implements: blueprint neutron-advanced-scenarios

Change-Id: If7cb1f093172ae4f480e4a408c7f128061ca1ba6
Merge "Refactor cross_tenant to security_groups_basic_ops"