Use newer native "write HTTP" plugin to send metrics to Librato, if configured.

Add a status vhost to apache, so that monit's http monitoring will work.
It doesn't particularly matter to the monit check what this vhost does
as long as it returns 200, but I thought it would be nice to use
apache's builtin status functionality.  Ideas cribbed from [1].  It
might also be possible to use monit's apache-status functionality to
alert on more sophisticated criteria, but this will do for now.

Open question: does collectd support apache-status? Might it also be
interested in this vhost?

Fixes #299.

[1] http://mmonit.com/wiki/Monit/MonitorApacheStatus

This commit moves the monitoring role to the bottom of site.yml so that
it is executed after all other roles.

This is needed because the monitoring role conditionally installs Monit
configuration file based on whether some other packages have been
installed or not (such as ZNC).

This patch also adds a comment to the "monitoring" entry within the
"roles" list and a `roles/monitoring/README.md` file telling users why
the monitoring role has to come last.

Resolves #284

The other services monitored by the default monit task should always be present except in very custom setups.

Resolves #255.

This change set builds collectd from source and configures it in one of
the following ways:

- If Librato credentials are present, collectd will be configured to
send data points to Librato using the collectd-librato plugin.

- If no Librato credentials are present, collectd will be configured to
write RRD files locally (/opt/collectd/var/lib/collectd/rrd by default).

Remove all configuration for MySQL and configure PostgreSQL as the main
database.

All *_mysql_* options have been changed to *_db_* options.

Postgres requires the database user to have a password in order to
connect via localhost. The db_admin_password option is used to set the
password of the admin user (usually postgres).

Also mailing to root (which gets forwarded to admin_email) for easier automated sorting

Based on playbook from https://github.com/phred/5minbootstrap