Add skip_if_unavailable to yumrepo resource

If the repository that the puppet_agent module deploys is unavailable, it breaks all yum interactions. It would be helpful if one could optionally enable the skip_if_unavailable yumrepo parameter so that yum transactions can continue if the puppet_agent repo is unavailable for any reason.

PR submitted: https://github.com/puppetlabs/puppetlabs-puppet_agent/pull/205