<!--- 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 -->

depth=dict(default=None, type='int'),
            clone=dict(default='yes', type='bool'),
            update=dict(default='yes', type='bool'),
            verify_commit=dict(default='no', type='bool'),
            gpg_whitelist=dict(default=[], type='list', elements='str'),
            accept_hostkey=dict(default='no', type='bool'),
            accept_newhostkey=dict(default='no', type='bool'),
            key_file=dict(default=None, type='path', required=False),
            ssh_opts=dict(default=None, required=False),
            executable=dict(default=None, type='path'),
            bare=dict(default='no', type='bool'),
            recursive=dict(default='yes', type='bool'),
            single_branch=dict(default=False, type='bool'),
            track_submodules=dict(default='no', type='bool'),
            umask=dict(default=None, type='raw'),
            archive=dict(type='path'),
            archive_prefix=dict(),
            separate_git_dir=dict(type='path'),
        ),
        mutually_exclusive=[('separate_git_dir', 'bare'), ('accept_hostkey', 'accept_newhostkey')],
        required_by={'archive_prefix': ['archive']},
        supports_check_mode=True
    )
    dest = module.params['dest']
    repo = module.params['repo']
    version = module.params['version']
    remote = module.params['remote']
    refspec = module.params['refspec']
    force = module.params['force']
    depth = module.params['depth']