Adds new test case to validate min_ttl equal to Zero

1) Refactoring - moving negative test cases to the
   dedicated class
2) "test_create_zone_ttl_zero" - create a Zone with TTL:0
   and make sure that the created Zone has TTL:0

Change-Id: I96022307ef1e8deb59015c284c5fadce70720c82
3 files changed