Fix failing persist_iptables test on RHEL7 and Fedora

This corrects the following failure:
```
  1) Puppet::Util::Firewall #persist_iptables when proto is IPv4 should raise a warning when exec fails
     Failure/Error: subject.persist_iptables(proto)
       Firewall[000 test foo] received :execute with unexpected arguments
         expected: (["/sbin/service", "iptables", "save"])
              got: (["/usr/libexec/iptables/iptables.init", "save"])
     # ./lib/puppet/util/firewall.rb:220:in `persist_iptables'
     # ./spec/unit/puppet/util/firewall_spec.rb:166:in `block (4 levels) in <top (required)>'
```
Otherwise, persist_iptables falls into the RHEL7 case and sets os_key to 'Fedora', which causes the executed command to be set incorrectly.