overwrite password files if they exist

overwrite password files if they exist

The .fdroid.*.txt password files are only meant to be a conduit for the
passwords, so blow them away everytime.  The canonical password is stored
in config.py.

It might makes sense to replace these files with env vars using
-storepass:env and -keypass:env.  I figured that the passwords are already
in a file, config.py, so adding more files in the same location with the
same perms would not increase the risk at all.