Re-order the methods in ssh.py so that methods needed for implementation are near and just above the relevant public methods.

Standard with the rest of the code base.

A couple of ssh cleanups

Make pipelining a connection variable

This is also peripheral to what _build_command needs, can be improved
and tested independently, and so makes more sense in a separate method.

This commit doesn't change any functionality (and I've verified that it
works with the various combinations: control_path set in ansible.cfg,
ssh_args adding or not adding ControlMaster/ControlPersist, etc.).

Checking for sshpass is peripheral to the calling code, so it's easier
to follow when the details are moved into a method.

SSH pipelining can be a significant performance improvement, but it will
not work if sudoers is configured to requiretty. With this change, one
could have pipelining enabled in ansible.cfg, but use sudo to turn off
requiretty in a separate play (or task) where pipelining is disabled:

    - hosts: foo
      vars:
          ansible_pipelining: no
      tasks:
        - lineinfile: dest=/etc/sudoers line='Defaults requiretty' state=absent
          sudo_user: root

(Note that sudoers has a complicated syntax, so the above lineinfile
invocation may be too simplistic for production use; but the point is
that a separate play can do something to disable requiretty.)

Clarify why we add -q only for ssh

Python 3: the StringIO module is gone

Fixes 17 failing tests on Python 3.4, restoring the 8 failures we had
last Friday.

Output proper JSON from vbox dynamic inventory contrib script

fix no self pointer out of class.

Also get pipelining working for people who look to chroot as an example
for their own connection plugins

Note: In the latest v2 API, action handles become but chroot doesn't
reliably handle become.  Maybe we need to add a has_become attribute
that the action can display an appropriate error.

This can happen in chroot environments that don't have all of /proc
available

Helps connection plugin implementors understand how to structure
exec_command()

Fixes #12526

Update outdated comment

Since #12165 was merged, hostnames are properly validated.

We now use "sudo -n" instead of "sudo -k && sudo …"

Clarified that .yml, .yaml, and .json are allowable file extensions for inventory vars files

Also strip UnsafeProxy off of low level srings and objects to ensure
they don't cause issues later down the road

Fixes #12513