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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]
```
location:
        description:
            - Resource location.
        type: str
    sku:
        description:
            - The snapshots SKU.
        type: dict
        suboptions:
            name:
                description:
                    - The sku name.
                type: str
                choices:
                    - Standard_LRS
                    - Premium_LRS
                    - Standard_ZRS
            tier:
                description:
                    - The sku tier.
                type: str
    os_type:
        description:
            - The Operating System type.
        type: str
        choices:
            - Linux
            - Windows
    creation_data:
        description: