Description of problem:
When using Horizon to create an instance and providing "Customization script" at "Configuration" tab, it is impossible to use a script bigger than 16K. While Nova API has a related limit, it is 64K.

Also, I am not sure if there is a need to enforce separate limits in two places: Nova will return HTTP 400 when asked to create an instance and user data is bigger than 64K.

Version-Release number of selected component (if applicable):
RHOSP 17.1

How reproducible:
Try to upload user-data bigger than 16K, but less than 64K (Nova limit) while creating an instance in Horizon

Actual results:
Impossible to create an instance

Expected results:
Horizon doesn't enforce its own limitations over user-data