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
diff --git a/designate_tempest_plugin/config.py b/designate_tempest_plugin/config.py
index c244b12..f4ca708 100644
--- a/designate_tempest_plugin/config.py
+++ b/designate_tempest_plugin/config.py
@@ -41,7 +41,7 @@
                default=360,
                help="Timeout in seconds to wait for an resource to build."),
     cfg.IntOpt('min_ttl',
-               default=1,
+               default=0,
                help="The minimum value to respect when generating ttls"),
     cfg.ListOpt('nameservers',
                 default=[],