<!--- 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 -->
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

```

<!--- HINT: You can paste gist.github.com links for larger files -->

##### 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

```

if isinstance(s.command, string_types):
            s.command = shlex.split(s.command)
        elif isinstance(s.command, list):
            invalid_items = [
                (index, item)
                for index, item in enumerate(s.command)
                if not isinstance(item, string_types)
            ]
            if invalid_items:
                errors = ', '.join(
                    [
                        '%s (%s) at index %s' % (item, type(item), index)
                        for index, item in invalid_items
                    ]
                )
                raise Exception(
                    'All items in a command list need to be strings. '
                    'Check quoting. Invalid items: %s.'
                    % errors
                )
            s.command = ap['command']
        elif s.command is not None:
            raise ValueError(
                'Invalid type for command %s (%s). '
                'Only string or list allowed. Check quoting.'
                % (s.command, type(s.command))
            )
        s.env = get_docker_environment(ap['env'], ap['env_files'])
        s.rollback_config = cls.get_rollback_config_from_ansible_params(ap)
        update_config = cls.get_update_config_from_ansible_params(ap)