More config options + missing rrdtool for web interface

[jedi4ever]

• Added way to also search a role to find the collectd_server
• make it clear this cookbook is not for adding plugins in it but you need to do it in your own
• add rrdtool plugin to make web work out of the box (it need to be configured to point to the correct datadirectory)
• Remove hardcoding of paths in templates
• Make example for case sensitive usage in plugin definitions
• Make http_port reconfigurable

[coderanger]

This should probably be null by default so as to not change behavior unless requested.