Allow sink to create multiple entries per instance

There are a few situations where this may be useful, for
example simultaneous creation of

<hostname>.example.org
and
<instance-id>.example.org

The code already existed to clean up multiple entries for
a given host, so this change is pretty simple.

Co-Authored-By: Zhang Gengyuan <stanzgy@gmail.com>
Change-Id: I8a410107d970d079f757f9d22c44485856239517

etc/designate/designate.conf.sample

@@ -220,6 +220,7 @@ debug = False
 #notification_topics = notifications
 #control_exchange = 'nova'
 #format = '%(octet0)s-%(octet1)s-%(octet2)s-%(octet3)s.%(domain)s'
+#format = '%(hostname)s.%(domain)s'
 
 #------------------------
 # Neutron Floating Handler
@@ -230,6 +231,7 @@ debug = False
 #notification_topics = notifications
 #control_exchange = 'neutron'
 #format = '%(octet0)s-%(octet1)s-%(octet2)s-%(octet3)s.%(domain)s'
+#format = '%(hostname)s.%(domain)s'
 
 ###################################
 ## Pool Manager Cache Configuration