plugins/Pgsql: add env.PGLABEL to customize title

If you are using the same plugin for several postgres instances, you can customize graph
title with the environment variable PGLABEL.

If you setup several postgres plugins like :

    postgres_pg91_size_mydb
    postgres_pg92_size_mydb

Your graph title display the same title for both :

    $ munin-run postgres_pg91_size_mydb config | grep graph_title
    graph_title PostgreSQL database size (mydb)

    $ munin-run postgres_pg92_size_mydb config | grep graph_title
    graph_title PostgreSQL database size (mydb)

This change in Pgsql.pm takes an environment variable in the graph title.

Example in the munin-node conf:

    [postgres_pg91_ *]
    env.PGLABEL Pg 9.1
    [postgres_pg92_ *]
    env.PGLABEL Pg 9.2

After that you have a different graph title for each plugin :

    $ munin-run postgres_pg91_size_mydb config | grep graph_title
    graph_title PostgreSQL database size Pg 9.1 (mydb)

    $ munin-run postgres_pg92_size_mydb config | grep graph_title
    graph_title PostgreSQL database size Pg 9.2 (mydb)

I have also tested with different configuration and without PGLABEL defined :

    $ munin-run postgres_pg91_size_mydb config | grep graph_title
    graph_title PostgreSQL database size (mydb)

By default this patch change nothing for plugins already installed and running.
But if user want customize graph_title they can use the new environment
variable.