Password generated by debian package can use illegal chars

When using the "Autogenerate PostgeSQL config", the password is generated with: cat /dev/urandom | tr -cd "[:alnum:]" | head -c 8
On some systems the "alnum" seems to be extended to some non-ascii characters, which makes the creation of the PostgreSQL nuxeo user fail.

Solution: replace [:alnum:] with [a-zA-Z0-9]