GUI-855: Add pid to images cache key, since each gunicorn worker has its own memory cache

Fixes https://eucalyptus.atlassian.net/browse/GUI-855

With this change, the memory usage of the WSGI server will likely increase significantly, so we'll need to keep an eye on memory usage if/when we merge this.  We may want to add a note in the workers setting of console.default.ini to warn of memory usage if the worker count is increased in the config.  The memory usage concern is most significant on the AWS side due to the large image sets there.