Update iptables-ng cookbook

This version no longer attempts to load iptables v6 when there is no
ipv6 enabled.
Update Webtatic repository for RHEL 7 support

The regular Webtatic repository has no packages for RHEL 7: they are
all in testing. We must therefore use the testing repository.

The RHEL 7 repositories also use a different key, which we define here.

Additionally, some dependencies have moved from Webtatic to EPEL, so we
add them to EPEL's list of includepkgs.
Include the CentOS repositories on RHEL 7

On RHEL 7, it seems that some *-devel packages are no longer available
in the base RHEL repositories.

However, those packages are available in CentOS 7's repositories, so we
use those instead.
Rearchitecture RHEL / CentOS repository handling

We have a very different repository configuration for RHEL 7 from what
we have on RHEL 6. Rearchitect the code to reflect this difference.
Add additional RHEL dependencies from CentOS
Add rrdcached init file when it's not there

Rrdcached has no init file in the CentOS 7 repositories (despite being
installed).

We reuse repoforge's rrdcached init file. We can't use repoforge's RHEL
7 packages, because they don't exist.

See:

https://github.com/repoforge/rpms/blob/master/specs/rrdtool/rrdcached.init
Override bug in MySQL installation cookbook

This MySQL installation cookbook has a bug that prevents us from
installing MySQL 5.5.

Override the attribute so that we can proceed.
Upgrade Apache Cookbook to 2.0.0, rearchitect web

Rearchitect our web recipe so that we depend more on the updated Apache
cookbook, and minimize the amount of custom code present here.