Add additional RHEL dependencies from CentOS

Additionally restore the proper rrdtool includepkgs (we need
perl-rrdtool, so we need to have *rrdtool*, not just rrdtool*).
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.

Also override Apache package version, given that the Deb PHP (Sury) PPA
has a different version than what's expected on Ubuntu 12.04 (2.4 vs
2.2)
Add build-essential
Use a custom directory for session data

This gives us more flexibility w.r.t. permissions (we can make sure
it's owned by the same user / group as Apache).

This change is needed become as of the upgrade of the ugprade to the
Apache Cookbook 2.0.0, we changed the group (it used to be www-data /
apache, now it's scalr) that the Apache process runs as.