<!--- 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 -->

update = module.params['update']
    allow_clone = module.params['clone']
    bare = module.params['bare']
    verify_commit = module.params['verify_commit']
    gpg_whitelist = module.params['gpg_whitelist']
    reference = module.params['reference']
    single_branch = module.params['single_branch']
    git_path = module.params['executable'] or module.get_bin_path('git', True)
    key_file = module.params['key_file']
    ssh_opts = module.params['ssh_opts']
    umask = module.params['umask']
    archive = module.params['archive']
    archive_prefix = module.params['archive_prefix']
    separate_git_dir = module.params['separate_git_dir']
    result = dict(changed=False, warnings=list())
    if module.params['accept_hostkey']:
        if ssh_opts is not None:
            if ("-o StrictHostKeyChecking=no" not in ssh_opts) and ("-o StrictHostKeyChecking=accept-new" not in ssh_opts):
                ssh_opts += " -o StrictHostKeyChecking=no"
        else:
            ssh_opts = "-o StrictHostKeyChecking=no"
    if module.params['accept_newhostkey']:
        if not ssh_supports_acceptnewhostkey(module):
            module.warn("Your ssh client does not support accept_newhostkey option, therefore it cannot be used.")
        else:
            if ssh_opts is not None:
                if ("-o StrictHostKeyChecking=no" not in ssh_opts) and ("-o StrictHostKeyChecking=accept-new" not in ssh_opts):
                    ssh_opts += " -o StrictHostKeyChecking=accept-new"
            else:
                ssh_opts = "-o StrictHostKeyChecking=accept-new"