Add mock definitions for EL4

This commit adds support for mocking on EL4.

In it's current incarnation, there are a few oddities.
  * EL 4 uses vault.centos since yo isn't setup with a proper RHEL
  * I've created a legacy_mock defined type because it's a very
    different template.
  * This will only work at PL, since it's pointed to neptune.