(QENG-1126) Allow setting IS_PE to the negative

Previously we had an environment variable "IS_PE" or "BEAKER_IS_PE" to
set whether or not we were using PE. Since then PE has become the
default. Module authors would like a similarly simple way via the ENV
to override the default and set the installation type as "FOSS".
The convention that has arisen is to use the negative in the IS_PE
environment variable (e.g. "IS_PE=no"). This brings that convention
into Beaker proper.

Original work done by @cyberious (Travis Fields).
(gh-456) Can't ssh root@127.0.0.1 via vagrant if root...

... login disabled in /etc/ssh/sshd_config

- add enable_root_login step by default
- clean up spurious 'end' when generating/using an extra disk on
  virtualbox instances
- tested clean locally
(QENG-1449) proxy package managers before configuration/validation...

... of hypervisor SUTs

- run proxy_package_manager for each hypervisor before
  configure/validate step
(QENG-1450) when docker options are already set they should...

... not be overridden

- as reported in https://github.com/puppetlabs/beaker/pull/509
- repackaged PR to meet contributor guidelines
Merge pull request #517 from anodelman/vagrant-maint

(gh-456) Can't ssh root@127.0.0.1 via vagrant if root...
Fix #519 Unable to ssh to docker container based on Ubuntu Trusty
Merge pull request #518 from anodelman/maint

(QENG-1449) proxy package managers before configuration/validation...
Merge pull request #520 from anodelman/docker

(QENG-1450) when docker options are already set they should...
(GH-320) Allow disabling of vbguest plugin auto-update

A common Vagrant plugin can cause considerably lengthier test runs
unless disabled in the Vagrantfile.

This patch allows disabling the "vb auto update plugin" via an
environment variable

Originally coded by @petems (Peter Souter)
  refactored by @justinstoller (Justin Stoller)
Merge pull request #524 from jalvarezsamayoa/feature/issue_519

Fix #519 Unable to ssh to docker container based on Ubuntu Trusty
(MAINT) Allow passing multiple hosts to copy_module_to
(MAINT) Fix bug connecting to windows vagrant host

When running multiple hosts in vagrant with an linux master and windows
agent it seems that vagrant has difficulty connecting to box when the
ssh_config file contains 127.0.0.1 for the host. This change fixes the
issue by replacing it with the host ip if it has been specified in the
nodeset.

This might need a little more manual testing.
Merge pull request #521 from justinstoller/maint/master/multiple_hosts_for_copy_module_to

(MAINT) Allow passing multiple hosts to copy_module_to
Merge pull request #525 from justinstoller/GH-320-disable_vbguest_plugin

(GH-320) Allow disabling of vbguest plugin auto-update
Merge pull request #515 from justinstoller/QENG-1126

(QENG-1126) Allow setting IS_PE to the negative
Merge pull request #421 from liamjbennett/vagrant_ssh_config

Fix bug connecting to windows vagrant host
(QENG-1074) Override mac and win url for `install_puppet`

Previously we always downloaded windows or osx installation packages
from http://downloads.puppetlabs.com.

This allows the specifying of special urls for windows and osx to
`Beaker::DSL::InstallHelpers#install_puppet`.