<!--- 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 -->

cmd = [git_path, 'submodule', 'foreach', git_path, 'rev-parse', version]
    (rc, out, err) = module.run_command(cmd, cwd=dest)
    if rc != 0:
        module.fail_json(
            msg='Unable to determine hashes of submodules',
            stdout=out,
            stderr=err,
            rc=rc)
    submodules = {}
    subm_name = None
    for line in out.splitlines():
        if line.startswith("Entering '"):
            subm_name = line[10:-1]
        elif len(line.strip()) == 40:
            if subm_name is None:
                module.fail_json()
            submodules[subm_name] = line.strip()
            subm_name = None
        else:
            module.fail_json(msg='Unable to parse submodule hash line: %s' % line.strip())
    if subm_name is not None:
        module.fail_json(msg='Unable to find hash for submodule: %s' % subm_name)
    return submodules
def clone(git_path, module, repo, dest, remote, depth, version, bare,