Allow overriding the default default_soa_expire

Designate upstream has a shorter default value of SOA expire as
86400s=1d than RFC 1912 suggestion (2-4 weeks). So allow tweaking the
default value per operational needs.

Change-Id: I668a4470be612e596ab4db73ece0c37269c88005
Closes-Bug: #1784272

src/config.yaml

@@ -78,3 +78,10 @@ options:
     type: boolean
     default: false
     description: Enables experimental admin API for Designate.
+  default-soa-expire:
+    type: int
+    default: 86400
+    description: |
+      Default SOA expire value in seconds to specify how long a
+      secondary will still treat its copy of the zone data as valid if
+      it can't contact the primary.

src/templates/mitaka/designate.conf

@@ -28,6 +28,9 @@ debug = {{ options.debug }}
 # Which networking API to use, Defaults to neutron
 #network_api = neutron
 
+# SOA expire (integer value)
+default_soa_expire = {{ options.default_soa_expire }}
+
 #-----------------------
 # RabbitMQ Config
 #-----------------------