powerdns: fix instanciation with empty slave_instance_list

This was causing "No JSON object could be decoded"

software/powerdns/instance-powerdns.cfg

@@ -120,7 +120,7 @@ monitor-password = $${monitor-publish-parameters:monitor-password}
 #####################
 # Power DNS Slave configuration
 #
-{% set slave_instance_list = json_module.loads(slapparameter_dict.get('extra_slave_instance_list', '')) %}
+{% set slave_instance_list = json_module.loads(slapparameter_dict.get('extra_slave_instance_list', '[]')) %}
 
 # Iter through slave list to prepare configuration
 {% for slave in slave_instance_list %}

software/powerdns/software.cfg

@@ -25,7 +25,7 @@ mode = 0644
 [template-powerdns]
 recipe = slapos.recipe.template
 url = ${:_profile_base_location_}/instance-powerdns.cfg
-md5sum = 5077ba344b641fa9703f9872a974d3d7
+md5sum = f0d87be6df84f23c811638ce9d5f60ea
 output = ${buildout:directory}/template-powerdns.cfg
 mode = 0644