Set repo priority to 'undef'

[chuman]

This should be set to undef for:
if $priority { ... }
to be ignored in repo/yum.pp

[skpy]

The yumrepo priority parameter accepts absent as a valid value, which will ensure that no priority line is present in the generated repo file.

Locally, I've added the following line to the yum repo spec test:

should_not create_package('yum-plugin-priorities')

And rake spec runs the tests successfully. This indicates to me that Puppet is handling absent similar to undef.

However, I've also run puppet apply with the following trivial manifest:

$foo = absent
if $foo {
  notice("FOO IS TRUE")
}

and received the following output:

Notice: Scope(Class[main]): FOO IS TRUE
Notice: Compiled catalog for vagrant-smerrill.dev in environment production in 0.02 seconds
Notice: Finished catalog run in 0.05 seconds

So Puppet appears to be treating the bare word absent as boolean true within the conditional evaluation.

TL;DR: absent may well be installing the yum-plugins-priorities package even though no priority line will be defined within the generated repo file. It's not clear to me why the spec test claim it's not being installed; but this PR is a better solution all around.

Thanks!