<!--- Verify first that your issue is not already reported on GitHub -->
<!--- Also test if the latest release and devel branch are affected too -->
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##### SUMMARY
<!--- Explain the problem briefly below -->
In docker_swarm_service module in ansible, if we are specifying mount option as tmpfs, it is required to set src. But for docker service create, src is not supported for tmpfs.
##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
docker_swarm_service

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
ansible 2.8.4
  config file = /home/vani/Projects/voody/ansible/ansible.cfg
  configured module search path = ['/home/vani/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/local/lib/python3.6/dist-packages/ansible
  executable location = /usr/local/bin/ansible
  python version = 3.6.8 (default, Aug 20 2019, 17:12:48) [GCC 8.3.0]

```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below
ANSIBLE_NOCOWS(/home/user/Projects/ansible/ansible.cfg) = True
DEFAULT_STDOUT_CALLBACK(/home/user/Projects/ansible/ansible.cfg) = yaml
```

##### OS / ENVIRONMENT
<!--- Provide all relevant information below, e.g. target OS versions, network device firmware, etc. -->


##### STEPS TO REPRODUCE
<!--- Describe exactly how to reproduce the problem, using a minimal test-case -->

<!--- Paste example playbooks or commands between quotes below -->
```yaml

```

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##### EXPECTED RESULTS
<!--- Describe what you expected to happen when running the steps above -->


##### ACTUAL RESULTS
<!--- Describe what actually happened. If possible run with extra verbosity (-vvvv) -->

<!--- Paste verbatim command output between quotes -->
```paste below

```

rollback_config_failure_action = (
        (client.module.params['update_config'] or {}).get('failure_action')
    )
    update_failure_action = client.module.params['update_failure_action']
    failure_action = rollback_config_failure_action or update_failure_action
    return failure_action == 'rollback'
def main():
    argument_spec = dict(
        name=dict(type='str', required=True),
        image=dict(type='str'),
        state=dict(type='str', default='present', choices=['present', 'absent']),
        mounts=dict(type='list', elements='dict', options=dict(
            source=dict(type='str', required=True),
            target=dict(type='str', required=True),
            type=dict(
                type='str',
                default='bind',
                choices=['bind', 'volume', 'tmpfs', 'npipe'],
            ),
            readonly=dict(type='bool'),
            labels=dict(type='dict'),
            propagation=dict(
                type='str',
                choices=[
                    'shared',
                    'slave',