cs_instance: fix AttributeError in get_details (#80)

Author: resmo

changelogs/fragments/79-cs_instance_fix_details.yml

@@ -0,0 +1,2 @@
+bugfixes:
+- cs_instance - Fixed custom service offerings usage (https://github.com/ngine-io/ansible-collection-cloudstack/issues/79).

plugins/modules/cs_instance.py

@@ -695,6 +695,9 @@ def get_details(self):
         cpu_speed = self.module.params.get('cpu_speed')
         memory = self.module.params.get('memory')
         if all([cpu, cpu_speed, memory]):
+            if details is None:
+                details = dict()
+
             details.extends({
                 'cpuNumber': cpu,
                 'cpuSpeed': cpu_speed,