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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]
```
try:
            response = self.mgmt_client.query(url=self.url,
                                              method='DELETE',
                                              query_parameters=self.query_parameters,
                                              header_parameters=self.header_parameters,
                                              body=None,
                                              expected_status_codes=self.status_code,
                                              polling_timeout=600,
                                              polling_interval=30)
        except CloudError as e:
            self.log('Error attempting to delete the Snapshot instance.')
            self.fail('Error deleting the Snapshot instance: {0}'.format(str(e)))
        return True
    def get_resource(self):