Improve readability of test_networks
Improve readability of test_networks

* Use list comprehension where it makes sense.

* Remove redundant assertions. That is, assertions in some tests that
  are really the responsibility of another test. For example, checking
  if a given network id is present in a list was being checked in both
  test_list_networks and test_list_networks_fields.

* Create assertEmpty and assertNotEmpty methods to help further clarify
  the intent of certain assertions.

Change-Id: Id91b2d2b6699eaf4acc332b3e76f9cb46c09d7c7

cneill/tempest-lib,JioCloud/tempest-lib,sorenh/tempest-lib,afaheem88/tempest-lib,mtreinish/tempest-lib,varunarya10/tempest-lib