(PUP-1362) (PUP-1775) Use yum check-updates for latest yum packages

Early versions of the yum package provider used `yum list available` to
determine the latest version of packages that were `ensure => latest`.
However the versions of yum available around that time changed the
output formatting of yum, which meant that the parsing code would break
on a regular basis (#836).

Commit 8722e439 resolved this by using the yum python bindings to
interact with yum. This required shipping a python helper script to
provide the interface between Puppet and the python libraries. However
this opened up a host of other issues ranging from what version of
Python is symlinked to '/usr/bin/python', how the yum database is
locked, and so on (PUP-1362, #11380, #16623). In addition changes to
yumhelper.py were very hard to test or reason about since it doesn't
neatly plug into our existing tests.

This commit removes yumhelper.py and replaces the update checks by
calling `yum check-update`. The output format of this command has not
changed since CentOS 4 and works all the way up to RedHat 7 so we should
be able to rely on this for a while.