packaging: setup: postgres provision: generate only ASCII password

python string module is locale aware, so generating letter in non
unicode locale will result non unicode password, unreadable by java.

instead of using the string module, use hardcoded known good characters.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1035932
Change-Id: I1b27d8eb26c777617ff3bf625b979a75bc3225c6
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>