Supports of multiple postgres installation per host for #1196

Allow you to monitor several postgresql instances. 
Just link to postgres_<tag>_size_<databasename|ALL>
The <tag> can be what you want but without "_". It's allow you to define several
database configuration.
Example :
```
    [postgres_pg91_*]
    env.PGPORT 5432
    [postgres_pg92_*]
    env.PGPORT 5433
```
After you can try with :
```
 munin-run postgres_pg91_size_ALL
 munin-run postgres_pg84_size_ALL
```
Probably it can be improved in pgsql.pm if anyone has any advice I am open.