Fix the handling of non-existing paste file

First only call find_file() by it's self so we can check for None
(abspath crashes if you pass it None)
We are already calling abspath at the return, so this
also prevents a double call to abspath.

If return None from _get_deployment_config_file() load_paste_app() will
print out a nice error message.

Change-Id: I451e090b7f08266ca824b5da228dafde55726d94