Make sure gpgcheck is properly set in repo file.

Fixes Bugzilla #1098241.

Since the katello-installer will try to install packages from the
internal repo created by the installer, it is necessary to properly
configure the 'gpgcheck' attribute in the repo file when packages are
not signed.
Merge pull request #33 from omaciel/disable-gpgcheck

Make sure gpgcheck is properly set in repo file.