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##### SUMMARY
<!--- Explain the problem briefly below -->
`ansible-inventory` is supposed to return JSON...
but it does not when installing anisble@devel, and running a GCE inventory update with this `gcp_compute.yml` to use the gcp inventory plugin:

```
auth_kind: serviceaccount
filters: null
plugin: gcp_compute
projects:
- myproject
service_account_file: creds.json
```

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
gcp inventory plugin
##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
 ansible-inventory --version    
ansible-inventory 2.9.0.dev0
  config file = /etc/ansible/ansible.cfg
  configured module search path = ['/home/elijah/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /home/elijah/venvs/py36towerqa/lib64/python3.6/site-packages/ansible 
  executable location = /home/elijah/venvs/py36towerqa/bin/ansible-inventory                            
  python version = 3.6.8 (default, Feb 19 2019, 21:10:40) [GCC 9.0.1 20190209 (Red Hat 9.0.1-0.4)] 
```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below
#no output
```

##### 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 -->
1) Create gcp_comput.yml and creds.json to access your gcp account
2) Create python36 venv
3) install anisble@stable-2.8
4) run `ansible-inventory -i gcp_compute.yml --list > ouput.28`
5) Inspect the output, see that it is valid JSON.
6) install ansible@devel
4) run `ansible-inventory -i gcp_compute.yml --list > ouput.devel`
7) Inspect output, see that before the part that starts the JSON that looks like:
```
{
    "_meta": {
        "hostvars": {
```
there is a bug chunk of key/value pairs in a list:
```
[{'key': 'blahkey', 'value': 'blahvalue}, {'key': 'ssh-keys', 'value': 'somebody's key'} {'key': 'more keys', 'value': 'more values' ...... ] 
```


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

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

##### ACTUAL RESULTS
<!--- Describe what actually happened. If possible run with extra verbosity (-vvvv) -->
List of key/value pairs then regular json ouput
<!--- Paste verbatim command output between quotes -->
```paste below

```
This is a tower blocker for the next release of ansible.
 (private repo) https://github.com/ansible/tower/issues/3605

if not HAS_GOOGLE_LIBRARIES:
            raise AnsibleParserError('gce inventory plugin cannot start: %s' % missing_required_lib('google-auth'))
        super(InventoryModule, self).parse(inventory, loader, path)
        config_data = {}
        config_data = self._read_config_data(path)
        if self.get_option('use_contrib_script_compatible_sanitization'):
            self._sanitize_group_name = self._legacy_script_compatible_group_sanitization
        
        params = {
            'filters': self.get_option('filters'),
            'projects': self.get_option('projects'),
            'scopes': self.get_option('scopes'),
            'zones': self.get_option('zones'),
            'auth_kind': self.get_option('auth_kind'),
            'service_account_file': self.get_option('service_account_file'),
            'service_account_contents': self.get_option('service_account_contents'),
            'service_account_email': self.get_option('service_account_email'),
        }
        self.fake_module = GcpMockModule(params)
        self.auth_session = GcpSession(self.fake_module, 'compute')
        query = self._get_query_options(params['filters'])
        if self.get_option('retrieve_image_info'):
            project_disks = self._get_project_disks(config_data, query)
        else:
            project_disks = None
        
        if cache:
            cache = self.get_option('cache')
            cache_key = self.get_cache_key(path)
        else: