Add unit on y-axis

Hi,

This pull request add on the left side of the y-axis of CPU, Load and Memory graphic a legend with unity.

It's perticulary usefull when you let some customer the ability to read your shinken webui. This reduce the miscomprehension.

I think, it will be a good idea to generalize this feature with all other graphite template packaged with pack.
I don't know if it's better the put this legend on the right side in order to increase readability on mobile device. What du you think about it ?

Feel free to comment, accept, refuse (or insult ?) this pull request !

Regards