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##### SUMMARY
<!--- Explain the problem briefly below -->
The snapshot module currently only supports storage blob as a creation source, it should
supports managed disk as a creation source. 
##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
lib/ansible/modules/cloud/azure/azure_rm_snapshot.py
##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
ansible 2.9.0
  config file = None
  configured module search path = [u'/home/haiyuan/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
  ansible python module location = /opt/ansible/local/lib/python2.7/site-packages/ansible
  executable location = /opt/ansible/bin/ansible
  python version = 2.7.12 (default, Oct  8 2019, 14:14:10) [GCC 5.4.0 20160609]
```
self.log("Old instance didn't exist")
            else:
                self.to_do = Actions.Create
        else:
            self.log('Snapshot instance already exists')
            if self.state == 'absent':
                self.to_do = Actions.Delete
            else:
                modifiers = {}
                self.create_compare_modifiers(self.module_arg_spec, '', modifiers)
                self.results['modifiers'] = modifiers
                self.results['compare'] = []
                self.create_compare_modifiers(self.module_arg_spec, '', modifiers)
                if not self.default_compare(modifiers, self.body, old_response, '', self.results):
                    self.to_do = Actions.Update
        if (self.to_do == Actions.Create) or (self.to_do == Actions.Update):
            self.log('Need to Create / Update the Snapshot instance')
            if self.check_mode:
                self.results['changed'] = True
                return self.results
            response = self.create_update_resource()
            self.results['changed'] = True
            self.log('Creation / Update done')
        elif self.to_do == Actions.Delete:
            self.log('Snapshot instance deleted')
            self.results['changed'] = True
            if self.check_mode:
                return self.results
            self.delete_resource()