installation: secret key creation fix

• Fixes problems with running inveniomanage config * without CFG_SITE_SECRET_KEY being manually set before. One is able to overwrite application config variable SECRET_KEY by passing equally named argument to application factory even if the global variable is not set.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>