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/data_utils.py b/designate_tempest_plugin/data_utils.py
index 61c6da0..629caa3 100644
--- a/designate_tempest_plugin/data_utils.py
+++ b/designate_tempest_plugin/data_utils.py
@@ -58,7 +58,7 @@
     return 'example@%s' % domain.rstrip('.')
 
 
-def rand_ttl(start=1, end=86400):
+def rand_ttl(start=0, end=86400):
     """Generate a random TTL value
     :return: a random ttl e.g. 165
     :rtype: string