<!--- 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 -->

'''
    Resets the index and working tree to HEAD.
    Discards any changes to tracked files in working
    tree since that commit.
    '''
    cmd = "%s reset --hard HEAD" % (git_path,)
    return module.run_command(cmd, check_rc=True, cwd=dest)
def get_diff(module, git_path, dest, repo, remote, depth, bare, before, after):
    ''' Return the difference between 2 versions '''
    if before is None:
        return {'prepared': '>> Newly checked out %s' % after}
    elif before != after:
        
        git_version_used = git_version(git_path, module)
        fetch(git_path, module, repo, dest, after, remote, depth, bare, '', git_version_used)
        cmd = '%s diff %s %s' % (git_path, before, after)
        (rc, out, err) = module.run_command(cmd, cwd=dest)
        if rc == 0 and out:
            return {'prepared': out}
        elif rc == 0:
            return {'prepared': '>> No visual differences between %s and %s' % (before, after)}
        elif err:
            return {'prepared': '>> Failed to get proper diff between %s and %s:\n>> %s' % (before, after, err)}
        else:
            return {'prepared': '>> Failed to get proper diff between %s and %s' % (before, after)}
    return {}
def get_remote_head(git_path, module, dest, version, remote, bare):