Fix password_file handling

The password_file attribute don't work. I described it in
the ticket https://tickets.puppetlabs.com/browse/MODULES-618?filter=-3

In the method to_pkcs12 only the password attribute is used.

I changed the password_file method ho read the password from password_file
and create always the tmp file, otherwise the call of keytool will fail because
the password needs to be two or three times in the tmpfile.