install correct packages for EL >= 7

this seems to do the trick for us; passed the iptables-ng integration tests, and passed our wrapper cookbooks' integration tests for rendering our specific rule-set on centos 7.

 there's also an iptables-utils package on EL 7, which currently only provides nfnl_osf with the pf.os database (OS fingerprinting database). I left it off for now, but say the word if you'd like that added as part of this PR.

testing result details below:
```bash
[nathwill@wyrd iptables-ng]$ rubocop
Inspecting 48 files
................................................

48 files inspected, no offenses detected
[nathwill@wyrd iptables-ng]$ foodcritic -f any .

[nathwill@wyrd iptables-ng]$ kitchen list 70
Instance                                 Driver   Provisioner  Last Action
lwrp-chain-create-default-centos-70      Vagrant  ChefZero     Verified
lwrp-chain-create-custom-centos-70       Vagrant  ChefZero     Verified
lwrp-chain-create-if-missing-centos-70   Vagrant  ChefZero     Verified
lwrp-chain-create-empty-centos-70        Vagrant  ChefZero     Verified
lwrp-rule-create-default-centos-70       Vagrant  ChefZero     Verified
lwrp-rule-create-custom-centos-70        Vagrant  ChefZero     Verified
lwrp-rule-create-custom-chain-centos-70  Vagrant  ChefZero     Verified
lwrp-rule-create-if-missing-centos-70    Vagrant  ChefZero     Verified
lwrp-rule-delete-centos-70               Vagrant  ChefZero     Verified
lwrp-rule-check-order-centos-70          Vagrant  ChefZero     Verified
recipe-default-centos-70                 Vagrant  ChefZero     Verified
recipe-install-centos-70                 Vagrant  ChefZero     Verified
attribute-enabled-tables-centos-70       Vagrant  ChefZero     Verified
```