<!--- Verify first that your issue is not already reported on GitHub -->
<!--- Also test if the latest release and devel branch are affected too -->
<!--- Complete *all* sections as described, this form is processed automatically -->

##### 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]
```
NoAction, Create, Update, Delete = range(4)
class AzureRMSnapshots(AzureRMModuleBaseExt):
    def __init__(self):
        self.module_arg_spec = dict(
            resource_group=dict(
                type='str',
                updatable=False,
                disposition='resourceGroupName',
                required=True
            ),
            name=dict(
                type='str',
                updatable=False,
                disposition='snapshotName',
                required=True
            ),
            location=dict(
                type='str',
                updatable=False,
                disposition='/'
            ),
            sku=dict(
                type='dict',
                disposition='/',
                options=dict(