(PUP-2271) Allow yumrepo proxy attribute to be set to _none_

Add fix and test to allow proxy to be set to _none_ to bypass global proxy
configuration for YUM.

See https://tickets.puppetlabs.com/browse/PUP-2271
Merge pull request #2767 from tayzlor/fix/master/PUP-2271

(PUP-2271) Allow yumrepo proxy attribute to be set to _none_