Adapt install.pp to the new interface of foreman::repos

[nbarrientos]

Since theforeman/puppet-foreman#943, foreman::repos expects to be passed a value for yum_repo_baseurl. In the Foreman module this is covered by a parameter of the public interface but puppet-foreman_proxy is not ready.

This must be addressed somehow, otherwise compilations will fail with:

Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Resource Statement, Foreman::Repos[foreman_proxy]: expects a value for parameter 'yum_repo_baseurl' ...