(PUP-2279) Add support for 'skip_if_unavailable' to `yumrepo`

As of Yum 3.2.26, Yum has had the ability to skip a defined repository
if unable to contact. Prior to this commit, there was no way to manage
this option on a repository configuration.

This commit adds support for the 'skip_if_unavailable' option to the
`yumrepo` core type.
(PUP-2279) Add tests for `yumrepo` type

This commit adds new spec tests for the 'skip_if_unavailable' property
supported by the `yumrepo` type.
Merge pull request #2515 from trlinkin/yumrepo

(PUP-2279) Add support for 'skip_if_unavailable' to `yumrepo`
Merge branch 'master' of https://github.com/puppetlabs/puppet