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.

fdroidserver/common.py

@@ -123,7 +123,7 @@ def write_password_file(pwtype, password=None):
     command line argments
     '''
     filename = '.fdroid.' + pwtype + '.txt'
-    fd = os.open(filename, os.O_CREAT | os.O_WRONLY, 0600)
+    fd = os.open(filename, os.O_CREAT | os.O_TRUNC | os.O_WRONLY, 0600)
     if password == None:
         os.write(fd, config[pwtype])
     else: