Ensure TTL is not 0 for zone imports

If zone imports have a ttl of 0 update
the zone ttl to the min_ttl configured for
the designate install

Change-Id: Idec854997fa9b6ad5d6af29bf6935c069dcbc40a
Closes-Bug: #1586178

designate/dnsutils.py

@@ -274,6 +274,8 @@ def from_dnspython_zone(dnspython_zone):
     soa = dnspython_zone.get_rdataset(dnspython_zone.origin, 'SOA')
     if soa is None:
         raise exceptions.BadRequest('An SOA record is required')
+    if soa.ttl == 0:
+        soa.ttl = cfg.CONF['service:central'].min_ttl
     email = soa[0].rname.to_text(omit_final_dot=True).decode('utf-8')
     email = email.replace('.', '@', 1)
     values = {