webfaf2: generate random secret key in %post

If /etc/faf/web2.conf contains @SECRET_KEY@ during installation
it's replaced by randomly generated alphanumeric string.

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>
Merge pull request #332 from abrt/gen_secret_key

webfaf2: generate random secret key in %post