Release 15.12

• This is a minor update release.
• Update Galaxy to the Galaxy 15.10 release.
• Preload the GIE IPyhton Docker container onto the image for faster startup.
• Quit storing tags in the cluster config to prevent persistent_data.yaml
  becoming too big when supplied as user data to worker instances.
• Updated library versions in requirements.txt.
• Add a more direct method for locating the Nginx configuration directory
  (thanks to @MatthewRalston)
• Made file system archive extraction more resilient and synchronous.