Module should support setting more than one DHCP listen interface

[antaflos]

The module theforeman/puppet-dhcp supports setting a list of DHCP listen interfaces, but this module only allows setting one. This can be an unwanted restriction on systems with multiple network interfaces where both a Foreman smart proxy and a DHCP server are managed.

A simple fix for specifying additional DHCP listen interfaces is the addition of new parameter foreman_proxy::dhcp_additional_interfaces which defaults to undef and can contain a string or a list of interface names that will be added to the DHCP listen interfaces of the contained dhcp class.

See PR #399.