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.