packaging: setup: set engine db credentials from config

engine-setup writes the engine database credentials to the dwh db
config file, but did not read them from there on the next run, instead
asking for them again.

Change-Id: I1e07b41d2834f74d4babe6429c2ba3979a4b12ed
Bug-Url: https://bugzilla.redhat.com/1156039
Bug-Url: https://bugzilla.redhat.com/1128787
Signed-off-by: Yedidyah Bar David <didi@redhat.com>