(PUP-3293) Remove deprecated and unused pson and rubygems features

This commit removes the deprecated `features.pson?` and `features.rubygems?`
features.
(maint) remove reference to ext/puppet-load.rb

In commit a8be1158d, the file `ext/puppet-load.rb` was deleted.
However, its reference for our packaging systems in
`ext/redhat/puppet.spec.rb`, causing our packaging on EL to fail.

This commit removes the reference in ext/redhat/puppet.spec.erb.
(maint) fix filename for acceptance sshkey

The boostrap script `acceptance/bin/ci-bootstrap-from-artifacts.sh` uses
an outdated filename/SSH key for our acceptance runs. We've been
(temporarily) hacking around this by sed'ing this file in our Jenkins
jobs. This commit fixes the filename in the script itself.
Merge pull request #3289 from rji/maint/stable/acceptance-sshkey

(maint) fix filename for acceptance sshkey
Merge remote-tracking branch 'upstream/stable'

* upstream/stable:
  (maint) fix filename for acceptance sshkey
Merge pull request #3288 from rji/maint/master/el-packaging-fixup

(maint) remove reference to ext/puppet-load.rb
(PUP-3272) Remove yaml deserialization acceptance tests now that we don't deserialize yaml
Merge pull request #3283 from whopper/pup-3293/removal/pson-rubygems

(PUP-3293) Remove deprecated and unused pson and rubygems features
(PUP-862) Use EL 7 masters for Windows acceptance

Prior to this commit, our Windows acceptance tests used RHEL 6 masters,
which ship with Ruby 1.8.7. As part of PUP-862, we need to bump these to
EL 7 to make use of a newer Ruby. We would also periodically run into
issues where there was a transient failure on the master that caused the
Windows acceptance job to fail.

This commit modifies all the Windows Beaker node configs to use RHEL 7
masters, and removes the 'agent' role from the masters.