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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]
```
)
                )
            ),
            state=dict(
                type='str',
                default='present',
                choices=['present', 'absent']
            )
        )
        self.resource_group = None
        self.name = None
        self.id = None
        self.name = None
        self.type = None
        self.managed_by = None
        self.results = dict(changed=False)
        self.mgmt_client = None
        self.state = None
        self.url = None
        self.status_code = [200, 201, 202]
        self.to_do = Actions.NoAction
        self.body = {}
        self.query_parameters = {}
        self.query_parameters['api-version'] = '2018-09-30'
        self.header_parameters = {}
        self.header_parameters['Content-Type'] = 'application/json; charset=utf-8'
        super(AzureRMSnapshots, self).__init__(derived_arg_spec=self.module_arg_spec,
                                               supports_check_mode=True,
                                               supports_tags=True)
    def exec_module(self, **kwargs):