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.

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).