Proposing fix for Issue #4324; adding support for su

Closed previous pull request #5244 for this issue due to some merge conflicts.  

Adds support to ssh and ssh_alt connection plugins.  The glaring omission here is lack of hostvars support, but that'll come in a future pull request unless you object to it not being here.  This also adds support to bin/ansible and bin/ansible-playbook, with --su, --su-user, --ask-su-pass CLI args.

Tests are passing.  

```
Ran 93 tests in 40.371s

OK (SKIP=1)
```