<!--- 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

```

#
from __future__ import absolute_import, division, print_function
__metaclass__ = type
ANSIBLE_METADATA = {'status': ['preview'],
                    'supported_by': 'community',
                    'metadata_version': '1.1'}
DOCUMENTATION = '''
---
module: docker_swarm_service
author:
  - "Dario Zanzico (@dariko)"
  - "Jason Witkowski (@jwitko)"
  - "Hannes Ljungberg (@hannseman)"
short_description: docker swarm service
description:
  - Manages docker services via a swarm manager node.
version_added: "2.7"
options:
  args:
    description:
      - List arguments to be passed to the container.
      - Corresponds to the C(ARG) parameter of C(docker service create).
    type: list
    elements: str
  command:
    description: