Adding new variable to allow monitoring of different pools

I run several PHP-FPM pools on my server to isolate different applications. In order to provide memory, processes and average process size data for each pool separately I've modified the plugins to include a new environment variable.

In order to use this, the munin-node file under plugin-conf.d has to include a new enviroment variable as follows:
```
[phpfpm_*]
env.phppool mypool
```