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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]
```
name=dict(
                        type='str',
                        choices=['Standard_LRS',
                                 'Premium_LRS',
                                 'Standard_ZRS']
                    ),
                    tier=dict(
                        type='str'
                    )
                )
            ),
            os_type=dict(
                type='str',
                disposition='/properties/osType',
                choices=['Windows',
                         'Linux']
            ),
            creation_data=dict(
                type='dict',
                disposition='/properties/creationData',
                options=dict(
                    create_option=dict(
                        type='str',
                        disposition='createOption',
                        choices=['Import'],
                        default='Import'
                    ),
                    source_uri=dict(
                        type='str',
                        disposition='sourceUri'