db: do not fallback if ENGINE_PGPASS is specified

existence of ENGINE_PGPASS within environment tells us to use a specific
file, do not fallback to anything else in this mode.

Change-Id: I0a92784c12d9fc8fcde6a57043c15cb2e7cc37db
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>