packaging: setup: Avoid engine DB password in answerfile if providing

If engine-setup is providing the DB the DB password is randomly
generated. Avoiding to write it in the answer file, the
password will be randomly choosed also replaying that answer file.

Change-Id: I3fb9d2c10d3809c68430084f6212eaa191d5ca21
Bug-Url: https://bugzilla.redhat.com/1139211
Signed-off-by: Simone Tiraboschi <stirabos@redhat.com>