OPENDJ-2557 - use a secure PRNG when generating password content

Use SecureRandom() when generating password content in order to make
generated passwords less predictable. On *nix systems this
implementation will consume data from /dev/urandom which is slower than
using a PRNG and may also block if there is insufficient entropy.
However, password generation is a relatively rare event, so the
potential performance implications can be ignored.