<!--- Verify first that your feature was not already discussed on GitHub -->
<!--- Complete *all* sections as described, this form is processed automatically -->

##### SUMMARY
<!--- Describe the new feature/improvement briefly below -->
The `ssh_opts` parameter relies on setting environment variables for git, such as GIT_SSH. Git did not recognize this environment variable until version 2.3.0 (see https://git-scm.com/docs/git/2.3.0), and many contemporary OSes (RHEL 7, CentOS7) include older versions of Git (1.8.3.1).

It would be useful to provide a warning in this case, similar to the warning for the `depth` parameter. E.g. "Your git version is too old to support the ssh_opts argument."

##### ISSUE TYPE
- Feature Idea

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

##### ADDITIONAL INFORMATION
<!--- Describe how the feature would be used, why it is needed and what it would solve -->
It would save users time when trying to troubleshoot failed git connections via SSH.

<!--- Paste example playbooks or commands between quotes below -->
```yaml
- name: clone a git repo
  git:
    repo: git@github.com:cherdt/hello_ansible.git
    dest: /opt/hello_ansible
    version: master
    ssh_opts: "{{-o IdentityFile=github_id_rsa}}"
```

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

changed = False
    if not os.path.exists(os.path.join(dest, '.gitmodules')):
        
        return changed
    gitmodules_file = open(os.path.join(dest, '.gitmodules'), 'r')
    for line in gitmodules_file:
        
        if not changed and line.strip().startswith('path'):
            path = line.split('=', 1)[1].strip()
            
            if not os.path.exists(os.path.join(dest, path, '.git')):
                changed = True
    
    if not changed:
        
        begin = get_submodule_versions(git_path, module, dest)
        cmd = [git_path, 'submodule', 'foreach', git_path, 'fetch']
        (rc, out, err) = module.run_command(cmd, check_rc=True, cwd=dest)
        if rc != 0:
            module.fail_json(msg="Failed to fetch submodules: %s" % out + err)
        if track_submodules:
            
            
            version = 'master'
            after = get_submodule_versions(git_path, module, dest, '%s/%s' % (remote, version))
            if begin != after:
                changed = True
        else: