Any process can read the process table, and can therefore see the entire
command line of any other process.  That means its a bad idea to ever put
passwords as part of a command line.  Python is executing keytool and
jarsigner command lines here, so now instead of putting the password on the
command line, a file is passed instead with suitable file permissions.
This should reduce the exposure a lot.  But still, sensitive passwords
should not be written to any text file.

This change requires OpenJDK-7 since the :file option to -storepass and
-keypass was only added in Java 7's keytool and jarsigner.