Add environment variable 'PRINTING_MAILHOST_FIXED_ADDRESS'.

This sends all emails to a single, fixed address.  PrintingMailHost
still needs to be enabled, so this is in addition to printing.

https://github.com/collective/Products.PrintingMailHost/issues/2
Merge pull request #3 from collective/maurits-fixed-address

Add environment variable 'PRINTING_MAILHOST_FIXED_ADDRESS'.