Add a config for a minimum ttl

This ensures we can specify the minimum ttl to use when creating zones
or recordsets from random ttls, which includes some refactoring to use
a single method for generating ttls.

Change-Id: I5c8620ff5a15b935ba075bec1f9750dbae4ffb70
diff --git a/designate_tempest_plugin/config.py b/designate_tempest_plugin/config.py
index 4094149..07e96b8 100644
--- a/designate_tempest_plugin/config.py
+++ b/designate_tempest_plugin/config.py
@@ -31,4 +31,8 @@
     cfg.IntOpt('build_timeout',
                default=60,
                help="Timeout in seconds to wait for an resource to build."),
+    cfg.IntOpt('min_ttl',
+               default=1,
+               help="The minimum value to respect when generating ttls"),
+
 ]