Fix 'dbname' in PostgreSQL Collector

Commit fixes 'dbname' which is missing from config in PostgreSQL Collector.
'dbname' can be specified in order to get list of databases in PostgreSQL by
monitoring user. 'dbname' can be different than 'postgres'.
'dbname' defaults to 'postgres' which is/was default behaviour.

Signed-off-by: Zdenek Styblik <zdenek.styblik@icflix.com>