Set a Configurable Minimum TTL

Enable a configuration value to set a sitewide minimum TTL
Implements: blueprint ttl-minimum

Change-Id: I8f3fff1e2a3ccf5bc744e5f18f41234e77151638

designate/exceptions.py

@@ -149,6 +149,11 @@ class InvalidRecordSetLocation(Base):
     error_type = 'invalid_recordset_location'
 
 
+class InvalidTTL(Base):
+    error_code = 400
+    error_type = 'invalid_ttl'
+
+
 class DomainHasSubdomain(Base):
     error_code = 400
     error_type = 'domain_has_subdomain'